Abstract:
Disclosed is a bifocal lens constructed such that the lens is not easily damaged when hit by flying fragments and, moreover, able to obtain a consistent focal length with a simple construction. The lens is equipped with a transparent substrate having transparency and rigidity, a transparent resilient body having transparency and elasticity, a transparent fluid enclosed in the space between the aforementioned transparent substrate and the aforementioned transparent resilient body, and a mechanism that induces changes in the shape of the aforementioned transparent resilient body. Changes are induced in the shape of the transparent resilient body to form the desired shape in the two states of the lens which comprises the transparent fluid enclosed in the space between the transparent substrate and the transparent resilient body, making it possible to obtain the desired characteristics (accurate optical characteristics) in either of the two states.

Description:
TECHNICAL FIELD 
     The present invention relates to a bifocal lens capable of varying its focal length and bifocal glasses provided with the same lens, and, to a bifocal lens applied suitably e.g., to glasses for elderly persons having a reduced focus-adjusting ability in the sense of sight and to magnifying glasses used when performing a delicate work at hand, and to bifocal glasses provided with the bifocal lens. 
     BACKGROUND ART 
     In case of a reduction of focus-adjusting ability in the sense of sight, there have conventionally been used glasses  10  as depicted in  FIG. 11  having a lens  11  for ordinary life and a lens  12  for seeing something nearby e.g., when reading a book or sewing, integrated into a single lens such that one of the two lenses is used depending on the angle of the line of sight of eyes so that an object can clearly be seen. 
     In other words, the user moves his/her eyeballs for use while being conscious of the upper side and the lower side so that an object of several meters or more in size or a scene is seen through the lens having its optical axis on the upper side and so that an object of the order of several tens of centimeters is seen through the lens having its optical axis on the lower side. 
     The user, however, needs to use the glasses while being conscious of, at all times, which lens of the upper and lower ones is being used. Further, in a case where careful attention is needed to peripheral objects while seeing an object at the center when doing a work nearby using the lower lens, the user has to follow the objects while turning his/her head since the angle of field is narrow. This results in a drawback that the user gets tired soon. 
     As a method for overcoming the drawback there has hitherto been proposed a variable-focus lens capable of adjusting its focal length depending on the distance to a target object. Proposed are ones (see, e.g., Patent Documents 1 and 2) having two transparent soft-elastic bodies with a transparent liquid or a gel-like substance confined therebetween so that the focal length is varied by changing the volume of the liquid or the gel-like substance; and one (see, e.g., Patent Document 3) having a transparent gel-like substance formed as a solid lens at one side and confined by a transparent soft-elastic film at the other side so that the focal length is varied by a deformation of the soft-elastic film. 
     It may be difficult, however, for the configuration of Patent Document 1 or 2 to obtain lens characteristics (accurate lens characteristics) as designed since the transparent soft-elastic bodies function as a lens under pressure. Further, in case of using as glasses, due to two faces made of the soft-elastic bodies, they may possibly often be deformed or damaged when touched by a finger or when hit by a flying fragment. It is thus necessary to cover at least one face in contact with the external world with a protective hood made of glass or plastic. Though Patent Document 3 solves the latter problem, difficulties still remain in control of the lens shape of the soft-elastic body and in acquisition of desired lens characteristics. 
     PRIOR ART DOCUMENT 
     Patent Documents 
     
         
         Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-249202 
         Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-14909 
         Patent Document 3: Japanese Laid-Open Patent Publication No. 2006-106488 
       
    
     SUMMARY OF THE INVENTION 
     First is to provide a structure hard to be damaged even if hit by an external flying fragment and second is to obtain a bifocal lens having a simple structure and a consistent focal length. 
     The invention relates to a bifocal lens comprising: a transparent substrate having a transparency and a rigidity; a transparent rigid-elastic body having a transparency and an elasticity; a transparent fluid confined to an area between the transparent substrate and the transparent rigid-elastic body; and a control mechanism inducing a change in the shape of the transparent elastic body wherein the transparent elastic body is a transparent rigid-elastic body combining a rigidity on a having a portion shaped concave or convex, the control mechanism is a drive mechanism for changing the portion from convex shape to concave shape or vice versa. 
     The bifocal lens comprises: a transparent substrate having a transparency and a rigidity; a transparent rigid-elastic body having a transparency and an elasticity; a transparent fluid confined to an area between the transparent substrate and the transparent rigid-elastic body; and a control mechanism periodically changing the shape of the transparent rigid-elastic body, wherein the transparent elastic body is a transparent rigid-elastic body combining a rigidity on a having a portion shaped concave or convex, the control mechanism is a drive mechanism for periodically changing the portion from convex shape to concave shape or vice versa alternately. 
     The invention includes bifocal glasses comprising a bifocal lens. 
     According to the invention, a change of the form of a transparent rigid-elastic body is induced so as to impart predetermined shapes to two different forms of a lens made up of a transparent fluid confined between a transparent substrate and the transparent rigid-elastic body, thereby enabling desired characteristics (accurate optical characteristics) to be obtained for both of the two forms. 
     According to the invention, in a lens made up of the transparent fluid confined between the transparent substrate and the transparent rigid-elastic body, desired characteristics (accurate optical characteristics) can be obtained for both of two different lens forms acquired by periodically changing the form of the transparent rigid-elastic body. 
     According to the invention, a bifocal lens is used to form glasses, as a result of which there can be obtained bifocal glasses allowing the user to use them without feeling any fatigue even when the user follows and looks hard at objects lying at different positions by virtue of use of the lens having a wider angle of field and accurate lens characteristics. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1(   a ) and  1 ( b ) are diagrams for explaining a principle of operation of a transparent rigid-elastic body that is one of constituent elements of the present invention. 
         FIG. 2(   a ) is a transverse cross-sectional view through a central portion of a plate-like disc of  FIG. 1 ; and  FIG. 2(   b ) is a conceptual diagram of a vertical displacement of the central portion when a force F is being applied. 
         FIG. 3  is an explanatory diagram of a basic way of thinking of a bifocal lens configured by use of a transparent elastic body among rigid-elastic bodies. 
         FIGS. 4(   a ) to  4 ( c ) are cross sections at the center of the transparent elastic body taking measures to cope with a liquid leak at the periphery. 
         FIGS. 5(   a ) to  5 ( d ) depict examples enabling various focal length combinations by imparting a lens effect to a transparent substrate or by differentiating the curvatures of the transparent substrate and of the transparent elastic body. 
         FIGS. 6(   a ) to  6 ( c ) are diagrams explaining a specific example for changing the transparent elastic body. 
         FIGS. 7(   a ) to  7 ( c ) are diagrams depicting an example in which the transparent elastic body is firmly secured at its peripheral portion C to a slider. 
         FIGS. 8(   a ) and  8 ( b ) depict an embodiment in which a force is electromotively imparted to the transparent elastic body. 
         FIGS. 9(   a ) and  9 ( b ) depict, byway of example, a device for driving a drive coil. 
         FIGS. 10(   a ) to  10 ( c ) are diagrams depicting an example of application to a head-mounted loupe using bifocal lenses. 
         FIG. 11  is a diagram for explaining an embodiment of a conventional bifocal lens and bifocal glasses. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     &lt;Explanation of Operation Principle of Elastic Rigid Body&gt; 
     An embodiment of the present invention will now be described with reference to the drawings. 
       FIGS. 1(   a ) and  1 ( b ) are diagrams for explaining a principle of operation of a transparent rigid-elastic body that is one of constituent elements of the present invention. In  FIG. 1 , reference numeral  100  denotes a disc made of a rigid elastic body. The disc  100  of the rigid elastic body is a plate-like disc having a diameter D and a thickness t that is obtained by cutting out a spherical part of a radius R (meanings of signs used herein will hereinafter be described). The rigid elastic body has a rigidity but also has an elasticity. Though not transparent, steel is common and it is well known that the steel is used as a leaf spring (by contrast, a flexible elastic body represented by rubber is referred to as a soft elastic body). A common glass falls as a transparent elastic body under the rigid-elastic body as long as it is used so as not to exceed its elastic limit. For use in the present invention, however, commonly-called plastics are convenient. Among them, polyvinyl chloride (PVC), polyethylene terephthalate (PET), U-polymer (trademark of UNITIKA Ltd.), etc., are preferred. First, as depicted in  FIG. 1(   a ), the plate-like disc  100  is placed in a downwardly convex manner with its rim  100   a  being secured, after which the plate is thrust up from under at a center  100   b  by a force F as indicated by an arrow so that the plate-like disc  100  rises while being gradually bent. When rising up to a certain level, it is inverted into an upwardly convex manner as depicted in  FIG. 1(   b ). Once inverted, its shape remains unchanged even though the force F is removed. From this state, the force F is applied in the opposite direction (from above) until the plate lowers a certain distance to recover its original state of  FIG. 1(   a ). 
     Referring then to  FIGS. 2(   a ) and  2 ( b ), the above status will be described in more detail.  FIG. 2(   a ) is a transverse cross-sectional view through a central portion of the plate-like disc  100  depicted in  FIG. 1(   a ); and  FIG. 2(   b ) is a conceptual diagram of a vertical displacement of the central portion when a force F is being applied. The contour of the plate indicated by a solid line is a shape that is obtained by cutting out part of a hollow spherical body with a radius R, the plate-like disc having a diameter D, a bulge T at the center relative to the peripheries, and a thickness t. Depicted is a case where the plate-like disc  100  indicated by the solid line is placed in a downwardly convex manner with its peripheral portions being pressed down from above, with its central portion being thrust up from below (the forces applied from both above and below are denoted by F: Note that the mass of the plate-like disc is negligible). The contour of the plate indicated by a dashed double-dotted line is a transverse cross section through the central portion of the plate-like disc in the state of  FIG. 1(   b ).  FIG. 2(   b ) depicts, as a conception, a relationship between the force F being applied and a vertical displacement y at the central portion. That is, the horizontal axis of the coordinates represents the force F applied and the vertical axis thereof represents the displacement y at the central portion of the plate-like disc. 
     Initially, the plate-like disc is free with no force F applied (F=0) and its central portion lies at point ‘a’ which is a position −T (y=−T) on the y-axis. When the force F is applied thereto as depicted in  FIG. 2(   a ), the central portion transitions as indicated by ( 1 ) of  FIG. 2(   b ). Namely, up to a position level with the rim, the central portion rises in proportion to the force F, and, when reaches point ‘b’ slightly (of the order of the thickness of the disc) exceeding the rim position, it rises sharply up to point ‘c’ while describing track ( 2 ), for stability. Within the region of track ( 2 ), the central portion transitions by only the internal stress of the plate-like disc without any external force. The stabilized state after the transition is the state of a downwardly facing plate (the shape of  FIG. 1(   b )) indicated by the dashed double-dotted line that is opposite to the upwardly facing plate state in the initial state depicted in  FIG. 2(   a ). 
     At this time, the plate-like disc is free with no force F applied (F=0) similarly to the initial state. The central portion in this state lies at point ‘c’, and, when a negative force F is being applied to the central portion, it descends down to a position level with the rim in proportion to the force F while describing track ( 3 ). Instantly reaching point ‘d’ slightly (of the order of the thickness of the disc) exceeding the rim position, the central portion sharply descends down to point ‘a’ while describing track ( 4 ) to restore its original state for stability. Similarly to the operation within the region of track ( 2 ), the central portion transitions within the region of track ( 4 ) by only the internal stress of the plate-like disc without any external force. 
     In this manner, this elastic body in the form of the plate-like disc has two stable shapes (upwardly concave plate shape and downwardly concave plate shape, both having a curvature of R). 
     &lt;Explanation of Operation Principle in Case of Application to Bifocal Lens&gt; 
       FIG. 3  is an explanatory diagram of a basic way of thinking of a bifocal lens configured by using, a transparent elastic body out of rigid-elastic bodies (hereinafter, referred to simply as elastic body) shown in the above operation principle. 
     A bifocal lens  200  is fabricated by preparing a plate-like disc transparent substrate  201  having a transparency and a rigidity and a plate-like disc transparent elastic body  202  having a transparency and a rigid elasticity; causing their convex surfaces to confront each other, with their rims coinciding with each other without leaving any spaces; and injecting and confining a transparent liquid into a space therebetween so that a lens function is produced. The rims of the transparent substrate  201  and of the transparent elastic body  202  are sealed up by a soft-elastic body  204  having an elasticity like rubber and having a generally annular shape with an English alphabetic C-shaped section like tires of automobiles and bicycles. 
     The configuration of the bifocal lens  200  will be described in more detail. 
     The transparent substrate  201  is made of a transparent resin (acrylics, polycarbonate, etc.) or of a transparent glass (which may be colored for use as sunglasses). Its thickness is of the order of 1 mm or more which is relatively thick (any thickness may be employed unless deformation occurs when a positive pressure or a negative pressure is applied to the liquid  203 ) so as to keep its rigidity. Although this may be used singly as a lens, the thickness is constant in the following description to facilitate the description of the essence of the present invention. The shape is a plate-like disc with its principal portion (portion as a lens transmitting light) having a uniform thickness, with its inner portion having a curvature of R, and with its rim having a cylindrical protrusion  206  for preventing the liquid  203  from leaking out of the gap between the substrate  201  and the transparent elastic body  202 . In case of using polycarbonate (PC) as a specific material, its light refractive index is 1.585, and when using BK7 that is an optical glass, its light refractive index is 1.518. 
     The transparent elastic body  202  is also made of a transparent resin and is much thinner than the transparent substrate  201  (The thickness is preferably of the order of 0.1 mm for acrylics, polycarbonate, etc., but it may be 0.2 mm or more when using a material such as PET or transparent vinyl chloride. Since this thickness is not absolute but is determined depending on the diameter D, it may be larger or smaller than the values shown hereinabove as long as the characteristics described in  FIGS. 1 and 2  are obtained). The transparent elastic body  202  is a plate-like disc having a uniform thickness and a curvature of R equal to that of the transparent substrate  201 . In case of using polycarbonate (PC) and acrylics, their light refractive indexes are 1.585 and 1.49, respectively, and, when using polyethylene terephthalate (PET), its light refractive index is of the order of 1.575. 
     The liquid  203  is a transparent liquid (which may be one with a high liquidity or oil-like one with a high viscosity, such as water, alcohol, spindle oil, and cedar oil, as long as the substance moves under pressure), and it may be colorless or colored if it transmits light in some degree. The light refractive index is 1.333 when using water, 1.362 in use of ethyl alcohol, and 1.516 in use of cedar oil. 
     The elastic body  204  is an elastic body that is soft like rubber and that has a generally annular shape with an English alphabetic C-shaped section like tires of automobiles and bicycles. The transparent substrate  201  and the transparent elastic body  202  are clamped between opposed ends of the C-shape under a constant force so that the positional relationship between the transparent substrate  201  and the transparent elastic body  202  becomes stable in either case where the transparent elastic body  202  is in the convex state (indicated by a solid line in  FIG. 4(   c )) or in the concave state (indicated by a dashed double-dotted line in  FIG. 4(   c )). 
     In the state (state  1 ), as depicted in  FIG. 3 , where the transparent elastic body  202  is downwardly convex as indicated by a solid line  202 - a , a double-convex lens having a curvature of R is formed by the three elements, i.e., the transparent substrate  201 , the transparent elastic body  202 , and the transparent liquid  203  confined therebetween. Then, the focal length f of the convex lens is f=R/(2×(n−1)) where n is refractive index of the liquid  203 . 
     Next, in the state (state  2 ) where the transparent elastic body  202  is upwardly convex as indicated by a dashed double-dotted line  202 - c , the gap between the transparent substrate  201  and the transparent elastic body  202  is unvarying at all portions on the entire surface so that three layers of the transparent substrate  201 , the transparent liquid  203 , and the transparent elastic body  202  integrally make up a transparent body with a certain thickness and a curvature R, as a result of which no lens effect occurs (that is, the focal length f at that time is infinite ∞). In other words, parallel light coming from a direction LL in  FIG. 3  passes through the transparent substrate  201 , the transparent elastic body  202 , and the liquid  203  and leaves intactly as parallel light. 
     If cedar oil is used as the transparent liquid  203  and the optical glass BK7 is used as the transparent substrate  201 , then no substantial reflection will appear at the boundary surface therebetween since their light refractive indexes are the order of 1.5 and rarely different from each other. Use of the polycarbonate (PC) and acrylics as the transparent substrate  201  also suppresses the reflection so as to be extremely little. Polycarbonate (PC) or polyethylene terephthalate (PET) having a higher modulus of elasticity is available as the transparent elastic body  202 , however the reflection at the interface is extremely little because their light refractive indexes are in the order of 1.5 or a little bit more. It is possible to make a lens having a high light transmission ratio by forming a light reflection prevention film on the interface, even when a substance such as water or ethyl alcohol which has a light refractive index much different from the light refractive indexes of the transparent substrate  201  and of the transparent elastic body  202  is used as the transparent liquid  203 . 
     The operation (method of causing the state to transition from the state  1  to the state  2  or of causing the state to transition from the state  2  to the state  1 ) of the thus constructed bifocal lens will be similarly described referring to  FIG. 3 . 
     Here, a force to cause a change in state is applied to the center and the periphery of the transparent elastic body  202 . In the description, the transparent substrate  201  is fixed as depicted in  FIG. 3  and a force F is applied to the center of the transparent elastic body  202  in order to cause a change from the state  1  to the state  2 , and the transparent substrate  201  is fixed just the same and a force F′ is applied to a peripheral portion z (ring shape) of the transparent elastic body  202  as shown in the diagram in order to cause a change from the state  2  to the state  1 . At this time, forces are neglected that the elastic body  204  exerts on the transparent substrate  201  and on the transparent elastic body  202 . 
     First, the initial state  202 - a  of the transparent elastic body  202  is a state where the transparent elastic body  202  is free and is subjected to no force F (F=0 and F′=0) (state  1 ). In this state, a convex lens is formed. From this state, when the transparent substrate  201  is fixed and a force F is applied continuously to a central portion ‘a’ of the transparent elastic body  202  from below in the diagram, the transparent elastic body  202  changes to a substantially central position  202 - b  indicated by a broken line (the transparent elastic body  202  at this time is in a substantially flat plate state), and once exceeds slightly the position  202 - b , the transparent elastic body  202  abruptly turns into a state  202 - c . That is, the central portion thereof rises up to the level of the rim in proportion to the force F, and once exceeds slightly (of the order of the disc thickness) the rim position, the transparent elastic body  202  turns to the state  202 - c  (downwardly facing plate state indicated by the dashed double-dotted line that is opposite to the upwardly facing plate state in the initial state) by only an internal stress of the plate-like disc without applying any external force (state  2 ). No lens effect arises at that time. 
     Next, the plate-like disc is in a state similar to the initial state where it is free and is not subjected to any force F (F=0 and F′=0). When a force F′ is applied continuously to the peripheral portion z of the transparent elastic body  202  in this state, the transparent elastic body  202  changes to the position  202 - b  indicated by the broken line (the transparent elastic body  202  at this time is in a substantially flat plate state), and once exceeds slightly the position, the transparent elastic body  202  turns abruptly to the state  202 - a . That is, the peripheral portion z rises up to the level of the central portion in proportion to the force F′, and once exceeds slightly (of the order of the disc thickness) the position of the central portion, the transparent elastic body  202  returns to the state  202 - a  (the upwardly facing plate state in the initial state (state  1 )) by only an internal stress of the plate-like disc without applying any external force (state  2 ). A convex lens is formed at that time. 
     Although the basic way of thinking is realized without change in the above embodiment, a high-accuracy assembling work is required at the points of contact at the peripheral portion between the transparent substrate and the transparent elastic body so as to prevent any leak of the transparent liquid. In changing the shape of the transparent elastic body between concave and convex repeatedly, the transparent liquid may possibly leak out as a result of abrasion. Thus, depicted in  FIG. 4  is an embodiment in which the peripheral portion is hermetically sealed so as not to bring about any liquid leak irrespective of a long time use. 
       FIGS. 4(   a ) and  4 ( b ) depict a transparent elastic body  302  coping with a liquid leak at the peripheral portion and are cross-sectional views through the center of the transparent elastic body  302 .  FIG. 4(   a ) is a diagram depicting the downwardly convex state, with a region A corresponding to  202  of  FIG. 3 . A region C is a portion securely adhered (bonded, fusion-welded, or clamped) to the transparent substrate though the region C will hereinafter be described in detail. A region B is a cushioning portion for providing a relief when an expansion force acts from the central portion toward the peripheral portion z in the process of the change of the plate-shaped region A from the convex state to the concave state and concave state to the convex state and for facilitating the movement of the peripheral portion z of the region A. The corrugated shape may otherwise be various shapes depending on the purposes.  FIG. 4(   b ) depicts a shape in the case where the same of  FIG. 4(   a ) is in its upwardly convex state. Although the regions A, B, and C are integrally formed herein, the region B may slightly be thinner than the region A to vary the rigidity, or the region A may be made of different members from the regions B and C, more specifically, the region A may be made of a rigid-elastic body and the regions B and C may be made of a soft-elastic body so that the purposes can be achieved more easily. 
       FIG. 4(   c ) is a conceptual diagram depicting a configuration in which, in place of the transparent elastic body of the bifocal lens of  FIG. 3 , the transparent elastic body described in  FIGS. 4(   a ) and  4 ( b ) is securely adhered to the transparent substrate so as to prevent a leak of the transparent liquid, with the replacing transparent elastic body being designated at  302 . 
     Reference numeral  301  denotes a transparent substrate corresponding to  201  of  FIG. 3 , reference numeral  303  denotes a transparent liquid corresponding to  203  of  FIG. 3 , and the transparent elastic body  302  corresponds to  202  of  FIG. 3 . In the region A,  302 - a ,  302 - b , and  303 - c  are depicted corresponding to  202 - a ,  202 - b , and  202 - c , respectively. To ensure the free motion of the peripheral portion z of the region A, the region B has an S-shaped section more winding than the corrugated shape depicted in  FIGS. 4(   a ) and  4 ( b ). The region C of the transparent elastic body  302  is securely adhered at a surface  304  to the peripheral portion of the transparent substrate  301 . In this manner, the transparent substrate  301  and the transparent elastic body  302  are hermetically sealed, with the result that the liquid  303  injected in the interior is prevented from leaking. 
     This bifocal lens  300  performs the same action as the action described in  FIG. 3  and can replace the bifocal lens  200  of  FIG. 3 . As a result, when the transparent elastic body  302  is in the state  302 - a , the lens acts as a convex lens, whereas when the transparent elastic body  302  is in the state  302 - c , no lens effect appears, and parallel light penetrate as it is. 
     &lt;Transparent Liquid and Displacement of Transparent Elastic Body Peripheral Portion Z&gt; 
     The displacement of the peripheral portion z is found when applying a force F or F′ causing a change in the state of the transparent elastic body of the bifocal lens as described above (assuming that it is a double-convex lens, the displacement is approximately equal to the thickness of one half of the lens). 
     It is shown herein together with major sizes of the lens when the transparent liquid used herein is water, ethyl alcohol, cedar oil, α-bromonaphthalene, and diiodomethane. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 water 
                 ethyl alcohol 
                 cedar oil 
                 α-bromonaphthalene 
                 diiodomethane 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 n = 
                 1.333 
                 1.333 
                 1.3618 
                 1.3618 
                 1.516 
                 1.516 
                 1.66 
                 1.66 
                 1.737 
                 1.737 
               
               
                 f = 
                 200 
                 250 
                 200 
                 250 
                 200 
                 250 
                 200 
                 250 
                 200 
                 250 
               
               
                 R = 
                 133.2 
                 166.5 
                 144.7 
                 180.9 
                 206.4 
                 258.0 
                 264.0 
                 330.0 
                 294.8 
                 368.5 
               
               
                 D1 = 
                 50 
                 50 
                 50 
                 50 
                 50 
                 50 
                 50 
                 50 
                 50 
                 50 
               
               
                 T1 = 
                 2.367 
                 1.888 
                 2.176 
                 1.736 
                 1.520 
                 1.214 
                 1.186 
                 0.948 
                 1.062 
                 0.849 
               
               
                 D2 = 
                 42 
                 42 
                 42 
                 42 
                 42 
                 42 
                 42 
                 42 
                 42 
                 42 
               
               
                 T2 = 
                 1.666 
                 1.330 
                 1.532 
                 1.223 
                 1.071 
                 0.856 
                 0.837 
                 0.669 
                 0.749 
                 0.599 
               
               
                   
               
               
                 where 
               
               
                 n = light refractive index 
               
               
                 f = desired focal length (unit: mm) 
               
               
                 R = radius of curvature of lens when it is double-convex 
               
               
                 D1 = lens effective diameter - Example 1 (unit: mm) 
               
               
                 T1 = lens thickness (one half) when lens effective diameter is D1 (unit: mm) 
               
               
                 D2 = lens effective diameter - Example 2 (unit: mm) 
               
               
                 T2 = lens thickness (one half) when lens effective diameter is D2 (unit: mm) 
               
             
          
         
       
     
     As can be seen from this, in case of cedar oil that is relatively easy to handle, when the focal length is 200 mm with the lens diameter of 42 mm, the lens net thickness is of the order of 2.14 mm, and the amount to displace by applying a force is of the order of 1.07 mm. Use of liquid having a large refractive index enables the displacement to be reduced. 
     &lt;Variations of Bifocal Lens&gt; 
     Although the embodiment has hereinabove been described in which using the transparent substrate with no lens effect and with a curvature equal to that of the transparent elastic body, two states are switched, one is the state  1  that is a convex lens state and the other is the state  2  that has no lens function (the focal length is infinite), various combinations of the focal lengths become feasible by imparting the lens effect to the transparent substrate or by causing the curvature of the transparent substrate to differ from that of the transparent elastic body. Some of the variations are depicted in  FIG. 5 . 
     A bifocal lens  310  depicted in  FIG. 5(   a ) is an example having the same shapes except the transparent substrate of the bifocal lens  300  of  FIG. 4(   c ) and using a transparent substrate  311  in the shape of a concave lens (note that the region C (see  FIG. 4(   a )) of the transparent elastic body  302  is securely adhered via a spacer ring  314  to the transparent substrate  311 ). By employing this configuration, a bifocal lens is obtained that has a positive focal length (i.e., a convex lens forms) in the state  1  and that has a negative focal length (i.e., a concave lens forms) in the state  2 . 
     Similarly, a bifocal lens  320  of  FIG. 5(   b ) is an example using a convex lens  321  as the transparent substrate of the bifocal lens  300  of  FIG. 4(   c ). By employing this configuration, a bifocal lens is obtained that has a positive focal length (i.e., a convex lens forms) in the state  1  and that has a positive focal length (i.e., a convex lens with different focal lengths forms) in the state  2  as well. 
       FIGS. 5(   c ) and  5 ( d ) are examples in which a transparent substrate  331  has a constant thickness similarly to the bifocal lens of  FIG. 4(   c ) but in which a spherical radius R 1  of the transparent substrate is not equal to a spherical radius R 2  of the transparent elastic body. 
     A bifocal lens  330  of  FIG. 5(   c ) is an example of R 1 &gt;R 2  in which a bifocal lens is obtained that has a positive focal length (i.e., a convex lens forms) in the state  1  and that has a negative focal length (i.e., a concave lens forms) in the state  2 . 
     A bifocal lens  340  of  FIG. 5(   d ) is an example of R 1 &lt;R 2  in which a bifocal lens is obtained that has a positive focal length (i.e., a convex lens forms) in the state  1  and that has a positive focal length (i.e., a convex lens with different focal lengths forms) in the state  2  as well. 
     The four examples described above are not exclusive as combinations of means for imparting a lens function to the transparent substrate and means for making the radius R 1  of the transparent substrate and the radius R 2  of the transparent elastic body different from each other. That is, by making the radius R 1  of the transparent substrate and the radius R 2  of the transparent elastic body different from each other in  FIGS. 5(   a ) and  5 ( b ), different focal lengths can be obtained in the states  1  and the state  2 . Alternatively, the transparent substrate may be formed as a convex lens or concave lens in  FIGS. 5(   c ) and  5 ( d ) so as to allow the focal lengths in the state  1  and the state  2  to differ from each other. 
     &lt;Structure to Change Focal Length&gt; 
     Although in case of using as a separate lens the forces F and F′ may be applied by hand to change the two states (states  1  and  2 ) of the bifocal lens depicted in  FIGS. 3 ,  4 ( c ), and  5 , individual manual applications of the forces will be inconvenient when the lens is mounted to a portion of any device such as glasses. A method of applying a force through electrical control will thus be described. 
     Referring to  FIGS. 6(   a ) and  6 ( b ), electrodes are added to the transparent substrate and the transparent elastic body of the lens having the same structure as that of  FIG. 4(   c ), with electric charges being applied to the electrodes so that Coulomb force gives rise to the force F or F′.  FIG. 6(   a ) is a cross-sectional view seen from the same direction as in FIG.  4 ( c ), depicting a transparent elastic body  402  in only the state  2  indicated by the dashed double-dotted line of  FIG. 4(   c ). A transparent substrate  401  and the transparent elastic body  402  are securely adhered at their respective surfaces  405  and  404  to each other by way of a spacer  406 . 
     Furthermore, an ITO thin film is formed as a transparent electrode  407  on the entire surface of the transparent elastic body  402  facing the transparent substrate  401 , while an ITO pattern is formed as transparent electrodes  408  and  409  on the side of the transparent substrate  401  facing the transparent elastic body  402 . The transparent electrode  408  is formed on the peripheral portion and the transparent electrode  409  is formed circularly on the central portion, the electrodes  408  and  409  being insulated from each other. The electrodes  408  and  409  are formed with their respective terminals T 1  and T 2 . When applying, with such a configuration, a minus potential to the transparent electrode  407 , a plus potential to the transparent electrode  408 , and a minus potential to the transparent electrode  409 , a repulsion acts on the central portion due to the minus-to-minus electric charges whereas an attraction acts on the peripheral portion due to the minus-to-plus electric charges, thereby generating a force equivalent to a force of F′. Conversely, when applying a minus potential to the transparent electrode  408  and a plus potential to the transparent electrode  409  with the transparent electrode  407  still remaining at the minus potential, the central portion is subjected to an attraction (F) due to the minus-to-plus electric charges whereas the peripheral portion is subjected to a repulsion due to the minus-to-minus electric charges, thereby generating a force equivalent to a force of −F′. 
     As another embodiment of driving method using the electrical control, a method will be described of generating an attraction or a repulsion by applying an electro magnetic induction field.  FIG. 6(   c ) depicts a state where a coil  415  of a transparent conductor is formed on the side of the transparent substrate  401  facing the transparent elastic body  402 . Reference numeral  416  is an outgoing line made of a transparent conductor extending from the innermost end of the coil  415  to the outside. Reference numeral  417  denotes a transparent film for electrically insulating the intersection between the outgoing line  416  and the coil  415 . The configuration is made in this manner to form electrodes U 1  and U 2  at the ends of the coil so that when current flows from the electrode U 1  to the electrode U 2 , a magnetic field maximized at the center is generated in a direction normal to the plane of paper. In the same manner, a coil  425  identical in shape to the coil  415  is formed on the side of the transparent elastic body  402  facing the transparent substrate  401 . An outgoing line  426  and an insulating film  427  correspond respectively to the outgoing line  416  and the insulating film  417 . If current is fed so as to generate an in-phase magnetic field on the pair of coils, an attraction F maximized at the center arises, whereas if current is fed so as to generate an opposite-phase magnetic field, a repulsion −F maximized at the center arises (the configuration as depicted in  FIG. 4(   c ) also generates the same attraction F and repulsion −F as long as are formed the coil  415  on the transparent substrate  301  and the coil  425  on the transparent elastic body  302 ). 
     Furthermore, the two methods may be effected at the same time to generate more forces. 
     For example, on the transparent elastic body  302  of  FIG. 4(   c ) is formed the electrode  407  in the shape of the coil  425 , while on the transparent substrate  301  are formed the electrode  408  remaining unchanged in shape and the electrode  409  in the shape of the coil  415  (the region to be shaped like the coil  415  is a region occupied by the electrode  409 ). Potentials are applied to the electrodes so that an attraction arises if plus-to-minus potentials and that a repulsion arises if plus-to-plus or minus-to-minus potentials. If plus and minus potentials are applied respectively to the electrode  407  shaped like the coil and to the electrode  409  shaped like the coil while simultaneously current is fed to the coils to generate an in-phase magnetic field, then an attraction takes place so as to be added to reinforce the attraction F. Conversely, if plus-to-plus or minus-to-minus potentials are applied while simultaneously current is fed to the coils to generate an opposite-phase magnetic field, then a repulsion takes place so as to be added to reinforce the repulsion −F. 
     Another embodiment electromotively inducing the forces F and F′ is then depicted in  FIG. 7 . This is a method (called a moving coil type) in which a ring-shaped electric coil (drive coil) is securely adhered to the peripheral portion of the transparent elastic body depicted in  FIG. 4 , which is placed in a certain magnetic field with electric current being fed, to thereby apply an external force thereto. 
     In the method of  FIGS. 7(   a ),  7 ( b ), and  7 ( c ), the peripheral portion in the region C of the transparent elastic body is securely adhered to a newly disposed slider instead of secure adhesion to the transparent substrate so that the slider is subjected to a force F′. 
     Reference numeral  501  denotes a transparent substrate corresponding to  201  of  FIG. 3 , reference numeral  503  denotes a transparent liquid corresponding to  203  of  FIG. 3 , reference numeral  502  denotes a transparent elastic body corresponding to  202  of  FIG. 3 , and reference numerals  502 - a  and  502 - c  are also depicted corresponding to  202 - a  and  202 - c , respectively. Reference numeral  506  denotes a protrusion corresponding to  206  of  FIG. 4 . Reference numeral  533  denotes a slider securely adhered to the peripheral portion C of the transparent elastic body  502 . Reference numeral  532  denotes an electrically-driven drive coil that is securely adhered via the slider  533  and a slider  534 . Reference numeral  531  denotes a permanent magnet having a U-shaped section and a generally ring-like shape. The magnet  531  is magnetized so that the extremities of the U shape act as the N-pole and the S-pole, respectively, with the slider  533  and the drive coil  532  being fitted in the inside of the U shape. Configuration is such that the outer edge of the U shape confronts the protrusion  506  of the transparent substrate  501 , with its confronting surface  535  being integrally securely adhered to the protrusion. Its integrated inner periphery allows the slider  533  to slides therealong. 
       FIG. 7(   a ) depicts a state where the transparent elastic body  502  is in the state  1 , with the drive coil  532  slightly extending beyond the extremities of the U shape of the permanent magnet  531 .  FIG. 7(   b ) depicts a state where the transparent elastic body  502  is in the state  2 , with the drive coil being fully received within the recess of the U shape of the permanent magnet  531 . It is natural that the volume of the transparent liquid  502  remains unchanged from  FIG. 7(   a ).  FIG. 7(   c ) depicts, in a perspective view, only the relationship among the permanent magnet  531 , the drive coil  532 , and the slider  533 . 
     When a positive current is fed to the coil  532  in such a configuration, a force F′ can be applied to the peripheral portion C of the transparent elastic body  502 , causing a change from the state  1  to the state  2 . When a negative current is fed, a force −F′ is applied to the peripheral portion C, causing a change from the state  2  to the state  1 . 
     In case of the configuration depicted in  FIGS. 7(   a ) and  7 ( b ), after the repeated use, the degree of hermetical sealing may be impaired and the liquid may leak out. Thus,  FIG. 8(   a ) depicts an example of obtaining a bifocal lens  560  through the electromagnetic coil driving described above with the sealing structure as depicted in  FIG. 4(   c ). Referring to  FIG. 8(   a ), the regions A and B of the transparent elastic body depicted in  FIG. 4  are separated from each other, with a rib protruding from the peripheral portion z of the portion A, the rib being fitted with the drive coil so as to generate a driving force F′. Describing in detail, a rib  512  with an L-shaped section is formed as an extension of the region A on the peripheral portion of the transparent elastic body  502 , and a portion corresponding to the region B is formed from a rigid elastic body  504  that is made of a material different from that of the region A and that is a ring with an S-shaped section, and the both are securely adhered at a portion  508 . Since the peripheral portion (the region C of  FIG. 4)  of the rigid-elastic body  504  is clamped by the transparent substrate  501  and the spacer  507  and therefore is completely sealed up by the transparent substrate  501 , the transparent rigid body  502 , and the rib  512  so as to prevent the transparent liquid  503  from leaking out to the exterior. A drive coil identical to the drive coil  532  depicted in the bifocal lens  500  is adhered to the rib  512 . The structure of the permanent magnet of the driving portion is the same as in the bifocal lens  500  (however, end faces of the two poles differ). 
       FIG. 8(   a ) is a single representation of the states depicted separately in  FIGS. 7(   a ) and  7 ( b ). That is, the state of action of the transparent elastic body as a result of the supply of current to the drive coil is quite the same as in the case of the bifocal lens  500 , and the state  1  is a state  502 - a  where the section is hatched and the state  2  is a state  502 - c  indicated by a dashed double-dotted line. 
       FIG. 8(   b ) depicts a case where the rigid-elastic body is replaced by a rubber-like soft-elastic body in the region B for the same purpose. The peripheral portion of the transparent elastic body  502  is formed as a rib  522  so as to clamp a soft-elastic body  514  between the rib and a hold-down ring  523  of L shape in section, the soft-elastic body  514  being securely adhered at portions  508  and  509 . Since the peripheral portion (the region C of  FIG. 4)  of the rigid-elastic body  514  is clamped by the transparent substrate  501  and the spacer  507  and therefore is completely sealed up by the transparent substrate  501 , the transparent rigid body  502 , and the rigid-elastic body  504  so as to prevent the transparent liquid  503  from leaking out to the exterior. The drive coil  532  is adhered to the L-shaped ring  523 .  FIG. 8(   b ) is also a single representation of the states depicted separately in  FIGS. 7(   a ) and  7 ( b ). 
     When a positive current is fed to the coil  532  in such a configuration, a force F′ can be applied to the peripheral portion of the region A of the transparent elastic body  502 , causing a change from the state  1  to the state  2 . When a negative current is fed, a force −F′ is applied to the peripheral portion of the region A of the transparent elastic body  502 , causing a change from the state  2  to the state  1 . 
     Although in  FIGS. 7 and 8  the electromagnetic coil is securely adhered to the edge of the transparent elastic body and current to the electromagnetic coil is turned on and off within the magnetic field of the permanent magnet, conversely a force F′ may be applied to the peripheral portion of the transparent elastic body by adhering a permanent magnet or a ring of soft steel (α-iron) to the edge of the transparent elastic body, replacing the U-shaped permanent magnet with an electromagnet, and applying and interrupting current blow to the electromagnet. 
     &lt;Coil Driving Embodiment&gt; 
     A method of switching the state between the state  1  and the state  2  by the electrical control will then be described more specifically using a moving coil type. 
       FIG. 9  depicts an example of a device for driving the drive coil  532 .  FIG. 9(   a ), SW 1  denotes a switch for changing over the mode between automatic and manual, the switch SW 1  selecting either a signal (manual) from a switch SW 2  or a signal (automatic) from an oscillator OSC. A potential change detector CD includes a delay unit DLY for delaying a change in potential of an input I by a certain time (td), two inverters INV 1  and INV 2 , and two AND gates AND 1  and AND 2 . An output III outputs a pulse of time duration td when the input I changes from low (L) to high (H), whereas an output IV outputs a pulse of time duration td when the input I changes from high (H) to low (L). AMP denotes an amplifier for driving the drive coil COIL ( 532 ), and since an output stage of amplifier is made up of complementary FETs no current flows through the coil COIL when the amplifiers AM 1  and AM 2  are both low (L) or high (H), a positive current flows through COIL when AM 1  is high (H) but AM 2  is low (L), and a negative current flows through COIL when AM 1  is low (L) but AM 2  is high (h). SW 2  denotes a switch that imparts a change signal to the input I by hand. The switch SW 2  gives rise to a change from low (L) to high (H) when turned off from on and a change from high (H) to low (L) when turned on from off. The oscillator OSC is an oscillator that generates a rectangular wave with a frequency F to automatically repeatedly impart a change signal to the input I. 
       FIG. 9(   b ) is a timing chart depicting waveforms at terminals corresponding to Roman numerals shown in  FIG. 9(   a ). V denotes a waveform of current flowing through the drive coil COIL ( 532 ), and VI denotes a position (therefore, a change in the focal length of the compound lens) of the transparent rigid-elastic body  502 , and denotes a position of s 1  denotes the state  1  and a position of s 2  denotes the state  2 . 
     An automatic mode will first be described. Although when the change-over switch SW 1  lies at a position b as depicted in  FIG. 9(   a ), a signal from the rectangular oscillator OSC is fed to the input I of the detector CD, the subsequent actions will be described referring to the timing chart of  FIG. 7(   b ). The oscillation waveform of the oscillator OSC is a rectangular wave that alternates between high (H) and low (L) at a given cycle T=1/F as in I. The delay unit DLY delays the waveform of I by a delay time td as in II. The AND gate AND 1  takes the logical AND of a waveform at I with a waveform obtained by inverting the waveform at II by the inverter INV 1 , output of which goes high (H) as in III during the delay time td from a rise at I to a rise at II. The AND gate AND 2  takes the logical AND of a waveform at II with a waveform obtained by inverting the waveform at I by the inverter INV 2 , of which output goes high (H) as in IV during the delay time td from a fall at I to a fall at II. The respective outputs are delivered to the amplifiers AM 1  and AM 2  so that current is fed with a waveform shown at V to the coil COIL (drive coil  532 ). That is, when III is high (H), the amplifier AMP 1  goes high (H) with the amplifier AM 2  remaining low (L) to feed a positive current (in the direction indicated by an arrow) to the coil COIL, whereas when IV is high (H), the amplifier AMP 2  goes high (H) with the amplifier AMP  1  being already low (L) to feed a negative current (in the direction opposite to the arrow) to the coil COIL. 
     In the bifocal lens  500  ( 560 ,  570 ) of  FIG. 7  ( FIG. 8 ), when current is fed in the direction of arrow to the drive coil (COIL) at that time, the transparent elastic body is drawn into the interior of the U-shaped section of the magnet  531 , with the result that the transparent elastic body changes from the state (a) through the state (b) to the state (c), allowing a change from the state  1  (the state of a convex lens: focal length f 1 ) to the state  2  (the state of no lens effect: focal length f 2 ). In this case, the delay time td is preferably set so as to exceed the time required for the transparent elastic body to pass through the state (b) from the state (a). 
     When current is fed in this state to the drive coil  532  (COIL) in the direction opposite to the arrow, the transparent elastic body is thrust out of the interior of the U-shaped section of the magnet  531 , so that the transparent elastic body changes from the state (c) through the state (b) to the state (c), allowing a change from the state  2  (the state of no lens effect: focal length f 2 ) to the state  1  (the state of a convex lens: focal length f 1 ). In this case, the delay time td is preferably set so as to exceed the time required for the transparent elastic body to pass through the state (b) from the state (c). 
     In this manner, the bifocal lens  500  ( 560 ,  570 ) can repeat the state of a convex lens and the state of no lens effect at a cycle T=1/F, so that if a user with farsightedness or presbyopia uses the glasses mounted with this lens, a distant scene is clearly image-formed on the user&#39;s retina for the duration of no lens effect state though it is dim for the duration of the convex lens state, whereas a nearby object is clearly image-formed thereon for the duration of the convex lens state though it is dim for the duration of the no lens effect state. This enables the user to clearly see a nearby object even if the user sees the object immediately after seeing a distant scene without performing any operations halfway, and, conversely, to clearly see a distant scene even if the user sees the scene immediately after observing a nearby object. 
     When the change-over switch SW 1  is then turned to the point a for manual operation, an on-to-off operation of the switch SW 2  causes a pulse of duration time td to be output from the output III of the detector CD only for that moment, while an off-to-on operation of the switch SW 2  causes a pulse of during time td to be output from the output IV of the detector CD only for that moment. The subsequent actions are the same as the case of the automatic operation described above, and hence the detail description thereof will be omitted. 
     In this manner, the bifocal lens  500  ( 560 ,  570 ) is able to obtain the convex lens state and the no lens effect state by hand, thereby providing a lens having a proper focal length depending on an object to be observed. 
     &lt;Embodiment of Bifocal Glasses&gt; 
     Use of two of the bifocal lenses as set forth hereinabove enables the construction of eyeglasses and loupe for far vision and near vision. 
       FIG. 10  depicts an embodiment of a head-mounted loupe using the bifocal lenses. 
       FIG. 10(   a ) is an external view of a head-mounted loupe  1000  made up of two of the bifocal lenses  560  depicted in  FIG. 8  (the lens  500  of  FIG. 7  or the lens  570  of  FIG. 8  may also be available). The two bifocal lenses  560  are coupled to each other via a bridge  110  and are securely adhered to sleeves  120  as well. The two sleeves  120  are coupled via a support shaft (not shown) to each other, with a support  140  bearing the support shaft such that when a loupe wearer moves the bifocal lenses  560  as indicated by an arrow J, the bifocal lenses  560  turn as indicated by an arrow R. The interior of the support  140  receives, together with a battery acting as a power source, an electric circuit for driving an electromagnetic coil, with the output of an coil driving amplifier connecting to the drive coils  532  for the bifocal lenses  560  by lead wires extending through the interiors of electric wire protecting pipes (made of a flexible resin, etc). A semiautomatic-automatic changeover switch is adhered to the loupe and a knob  145  slides as indicated by an arrow K. 
     A posture detection switch  160  (not shown in this diagram) is fitted in a storage box  175 . A body  170  is formed integrally with the support  140  and has a band  180  fastened thereto so as to enable its extension and withdrawal in the directions indicated by arrows B to adjust the length of the band. The band  180  has a spring property and is fitted with a power switch  185  that is turned on by a slight pressure applied to the wearer&#39;s head when mounted on the head. 
     When the user of such a head-mounted loupe applies the body  170  to his/her forehead and winds the band  180  around his/her head like a headband, the two bifocal lenses  560  come immediately in front of right and left eyes (even though the user wears other glasses at that time, the glasses and the bifocal lenses are together available with their lens surfaces superimposed). This mounting turns on the power switch  185 . 
     Referring to  FIG. 10(   b ), the structure of the posture detection switch  160  will be described. 
     A metal ball is received within the interior of a curved metallic pipe  161  so as to freely roll through the interior of the pipe. At the both ends of the pipe, terminals  162  and  163  made similarly of metal are supported by insulating covers  165  and  166 . When the metal ball  164  lies at the right end with the left side of the pipe raised as shown in the diagram, this state is called a horizontal state in which the metal ball  164  comes into contact with the metallic terminal  162  so that the metallic pipe  161  and the terminal  162  become short-circuited. When the entirety is then tilted by an angle θ or more, the left end is relatively lowered so that the metal ball  164  rolls through the interior of the pipe  161  to move to the left end to come into contact with the terminal  163 , consequently allowing the metallic pipe  161  and the terminal  163  to become short-circuited. 
     In other words, an electric switch is provided that turns on the terminal  162  and the pipe  161  with the horizontal state and that turns on the terminal  163  and the pipe  161  when the entire switching device tilts by the angle of θ or more. 
     The angle θ is set to be smaller than the angle of difference between the tilt angle of the head when the loupe wearer performs a dose-range activity with his/her head down and the tilt angle of the head when the loupe wearer faces straight ahead to look into the distance. The switch  160  is fitted in the storage box  175  such that the terminal  162  and the pipe  161  become short-circuited during the work but that the terminal  163  and the pipe  161  become short-circuited when looking into the distance. 
       FIG. 10(   c ) depicts an example of an electric circuit that drives the drive coil  531  of the head-mounted loupe. The detailed description of part overlapping with the description of  FIG. 9  will be omitted. In  FIG. 10(   c ), a posture detector SA detects a posture of the wearer&#39;s head and semiautomatically generates a pulse that goes high (H) for a certain time duration (td). Reference numeral SW 4  denotes the posture detection switch  160 , with the pipe  161 , the terminal  162 , and the terminal  163  corresponding respectively to terminal n, terminal a, and terminal b. FF denotes a flip flop including two NAND gates combined. MM 1  and MM 2  denote monomulti-vibrators each generating a pulse that goes to high (H) for a certain time duration (td) when the input changes from high (H) to low (L). The actions of the entire SA are such that an output III′ outputs a pulse when the switch SW 4  turns from the terminal a to the terminal b and that an output IV′ outputs a pulse when turning from the terminal b to the terminal a. A switch SW 3  is a switch for selecting whether to feed a pulse train generated by the oscillator OSC to the detector CD, the switch SW 3  being configured such that when the switch SW 3  is turned off or opened, the gate GATE is closed preventing the pulse train from the oscillator OSC from entering the detector CD. When the switch SW 3  is turned on or closed, the gate GATE is opened to allow a rectangular wave pulse train generated from the oscillator OSC to enter the input I of the detector CD so that in the same manner as in  FIG. 9(   a ), the output III outputs a pulse (pulse width td) when the input I changes from high (H) to low (L), whereas the output IV issues a pulse (pulse width td) when changing from low (L) to high (HL). An OR gate OR 1  allows signals from both the outputs III′ and III to pass therethrough as they are and an OR gate OR 2  allows signals from both the outputs IV′ and IV to pass therethrough as they are, for the delivery to the amplifiers AMP  1  and AMP 2 . 
     Afterward, in the same manner as in  FIG. 9(   a ), COIL  1  (a drive coil  532 - 1 ) and COIL 2  (ad drive coil  532 - 2 ) are energized. The drive coils  532 - 1  and  532 - 2  are represented applying the drive coil  532  of the separate bifocal lens  560  to both the left and right of the glasses. These actions are quite identical to the contents depicted in the timing chart of  FIG. 9(   b ) (the power source and the power switch  185  are omitted in this diagram). 
     Although an object can continuously be observed if the oscillator has a frequency of several Hz or more, the user may perceive a change between the two states up to of the order of 20 Hz. If more, however, the user can use the glasses without being conscious of the change between the two states. Since the movie has a frame rate of 24 Hz, more than 24 Hz would be effective, but the television has a frame rate of 30 Hz and therefore it would not be desirable to be equal thereto. Here about 27 Hz (T≈37 milliseconds) intermediate therebetween will be employed for description. 
     &lt;Description of Actions Upon Mounting Head-Mounted Loupe&gt; 
     Actions of the loupe will be described for the semiautomatic mode and the full-automatic mode through a scene where the thus configured head-mounted loupe  1000  is mounted on the head and actually used by a user (a person having presbyopia who does not need glasses when watching the landscape or television but who needs glasses or a loupe when reading a book or a newspaper or doing some work nearby). 
     The semiautomatic mode is convenient, for example, to read the newspaper while watching television in a living room, etc. 
     When the user mounts the loupe on his/her head, the power switch  185  is turned on and the electric circuit  150  becomes ready to work. The user adjusts the direction of J so that the bifocal lenses  560  come immediately in front of his/her eyes, and then operates the switch knob  145  to turn off the switch SW 3  (entering the semiautomatic mode). As a result, a pulse train generated from the oscillator OSC is blocked by the gate GATE so that no pulse appears from the outputs III and IV of the detector CD. 
     Since the user hangs his/her head somewhat down when reading the newspaper, the terminal  162  and the pipe  161  of the switch  160  are short-circuited via the metal ball  164 . That is, since the switch SW 4  is flipped onto the terminal a side, a pulse of time duration td is output from the output IV′ of the posture detector SA so that outputs from the amplifiers AM 2 - 1  and AM 2 - 2  go high (H) with the outputs from the AM 1 - 1  and AM 1 - 2  remaining low (L), while a current opposite to the direction of the arrow is fed to COIL 1  (the drive coil  532 - 1 ) and COIL 2  (the drive coil  532 - 2 ) to thereby put the transparent elastic body  502  of the lens body  560  into the state  1 , giving rise to a convex lens to enable the user to clearly read the characters or letters on the newspaper. When the user tries to look at the television screen in the process of reading or after finishing reading, the user raises his/her head to see the screen and the terminal  163  and the pipe  161  become short-circuited via the metal ball  164 . That is, since the switch SW 4  changes over from the terminal a side to the terminal b side, a pulse of time duration td is output from the output III′ of the posture detector SA so that outputs from the amplifiers AM 1 - 1  and AM 1 - 2  go high (H) with the outputs from the AM 2 - 1  and AM 2 - 2  remaining low (L), while a current in the direction of the arrow is fed to COIL 1  (the drive coil  532 - 1 ) and COIL 2  (the drive coil  532 - 2 ) to thereby turn the transparent elastic body  502  of the lens body  560  from the state  1  to the state  2 , giving rise to no lens effect to enable the user to clearly look at the television screen. 
     When the user takes up or operates an object lying at a some distant place (at a place 20 to 30 cm distant from the eyes) while doing a delicate work closely at hand (at a place of the order of 50 to 60 cm distant), there occurs a need for the user to look alternately at the object lying at a near place and the object lying at a far place, with the tilt angle of the head being small both upon seeing the near place and upon seeing the far place, and therefore the automatic mode will be convenient. 
     When the user mounts the loupe on his/her head, the power switch  185  is turned on and the electric circuit  150  becomes ready to work. The user adjusts the direction of J so that the bifocal lenses  560  come immediately in front of his/her eyes, and then operates the switch knob  145  to turn on the switch SW 3  (entering the automatic mode). The gate GATE is then opened to allow a pulse train generated by the oscillator OSC to be fed through the input I to the detector CD so that the outputs III and IV output a pulse train as depicted in the timing chart of  FIG. 9(   b ). At the same time, as depicting in the timing chart, COIL 1  (the drive coil  532 - 1 ) and COIL 2  (drive coil  532 - 2 ) are activated allowing the transparent elastic body to perform a repetitive action alternating the state  1  (s 1 ) with the state  2  (s 2 ). At this time, when the user looks at a nearby object, the object forms a blurred image at the timing of no lens effect in the state  2  (s 2 ) but a clear image at the timing of a convex lens in the state  1  (s 1 ) (human eyes become conscious of a clear image only while neglecting a blurred image). 
     That is, since the user can clearly capture an image of the object for about half the time, there is no hindrance to doing a work. Halfway, when the user tries to take up an object lying at a some distant place and turns his/her eyes thereto, the image of the object becomes blurred at the timing of a convex lens state in the state  1  (s 1 ), but the object can clearly be seen at the no lens effect timing in the state  2  (s 2 ). Similarly, the user can clearly capture an image of the object for about half the time, and hence when trying to take up an object, the user can catch the object without hindrance. In this manner, the user can see a nearby object and a some-distant object with their respective clear images without needing any troublesome works such as raising or lowering the lens portions of the loupe, thus leading to an improved work efficiency. 
     Thus, the semiautomatic use can reduce the opportunity to change over the focal length so that less current is fed to the drive coil to suppress the battery consumption, whereas the automatic use enables any objects to clearly be seen even when the displacement of the user&#39;s head is small due to a narrow range of disposition of the objects to be observed. 
     Although the embodiment in which the bifocal lenses are incorporated into a general-purpose loupe has hereinabove been described, a comfortable daily life level is ensured for the users by fabricating and wearing glasses suitable to the individual users&#39; characteristics. Namely, to fabricate glasses, a lens most suitable to the respective eye characteristics has only to be selected from among the variations of the bifocal lens as depicted in  FIG. 5 . This enables applications not only to presbyopia but also to correction of nearsightedness and farsightedness. 
     Naturally, using such a bifocal lens to fabricate a loupe or glasses enables a single loupe or a single pair of glasses to deal with a situation as well where a user sees both a nearby object and a relatively distant object, the user being a person who was nearsighted in youth but has come to have presbyopia with age or a person who was farsighted in youth but thereafter has come to have presbyopia. For an astigmatic person, by subjecting the transparent substrate to an optical treatment for astigmatism (which is a known technique using an inverse function of the astigmatic characteristics), a single loupe or a single pair of glasses is available both when seeing a nearby object and when seeing a relatively distant object in the case as well where the person has come to have presbyopia. 
     EXPLANATIONS OF REFERENCE NUMERALS 
     
         
         
           
               100  . . . plate-like disc of rigid elastic body 
               200 ,  300 ,  500 ,  560  . . . bifocal lens 
               201 ,  301 ,  311 ,  311 ,  401 ,  501  . . . transparent substrate 
               302 ,  502  . . . transparent elastic body 
               203 ,  303 ,  503  . . . transparent liquid 
               310 ,  320 ,  330 ,  340  . . . bifocal lens 
               407 ,  408 ,  409  . . . transparent electrode 
               504  . . . rigid-elastic body 
               507  . . . spacer 
               512 ,  522  . . . rib 
               531  . . . permanent magnet 
               532  . . . electromagnetic coil 
               1000  . . . head-mounted loupe