1.  Field of the Invention
The present invention relates to a projection apparatus and more particularly to a projection apparatus for forming on a plane of projection a mirror image of an object at unit magnification. 
2.  Description of the Prior Art
The projection apparatus for forming a mirror image on a projection plane is widely used in copying apparatus and other apparatus containing a transferring step. 
The projection apparatus used in the conventional copying machine generally uses a single projection lens system.  However, the use of one single projection lens system has a disadvantage that since its conjugate is relatively long, the size of the copying machine employing it becomes inevitably large.  To overcome this drawback there has already been proposed such a projection apparatus in which a plural number of lens systems each having a relatively short conjugate are disposed so as to make the individual lens systems take charge of the respective portions of one object to be projected.  A complete projection image of the whole object is formed on a projection plane by a composition of these partial images through the plural lens systems. 
One example of such a projection apparatus is disclosed in U.S. Pat.  No.  3,584,952 granted on June 15, 1971.  In the projection apparatus of this patent there are used a plurality of lens systems disposed in rows along the length of each strip area of an original to be copied, so that these lens systems may form, on a photosensitive medium, partial images corresponding to the plural strip areas of the original respectively. 
The use of such type of projection apparatus permits the construction of a relatively compact copying machine. 
The present invention relates to an improvement in this type of projection apparatus.  The improvement made by the present invention resides in that the number of lenses required for composing each lens system is reduced as compared with the apparatus disclosed in the above mentioned patent. 
Each lens system used in the projection apparatus disclosed in the above mentioned U.S. Patent is composed of three lenses, that is, front, middle and rear lenses.  The front lens serves to form an intermediate image of the corresponding portion of an original on the middle lens interposed between the front and rear lenses.  The intermediate image formed on the middle lens is reformed on a photosensitive material through the rear lens.  In this system, the middle lens functions only as a field lens and has no relation with the function of projection per se in the nature of lens.  However, this field lens is of importance in the point that it keeps the projected image on the photosensitive material at a uniform brightness. 
According to the present invention, the number of lenses in an unit lens system is reduced by employing a first and a second lens each having a length along the optical axis larger than the effective diameter thereof. 
Such lens the length of which along the optical axis is larger than the effective diameter thereof is disclosed in British Pat.  No.  954,629 the complete specification of which was issued on Apr. 8, 1964.  But, the apparatus in which such lens is used, is of the type which projects a partial image of object through one single lens system.  This apparatus disclosed therein is essentially different from that of the present invention according to which lens system arrays are made so as to form a composite image of an object from many partial images formed by the individual lens systems.  Moreover, the lens disclosed in the British Patent is neither a telecentric lens system nor a lens for forming an intermediate image between the first and second lenses.  The telecentric feature and intermediate image forming ability constitute one of the essential features of lens system according to the present invention. 
Japanese Patent Publication No.  30787/1970 has disclosed a projection lens system which forms a composite image from many partial images through lens systems disposed in arrays and which also has a telecentric arrangement.  However, this projection lens system is of ordinary type and the length of the lens measured along the optical axis is not longer than the effective diameter thereof, which is entirely different from that of the present invention.