This invention relates to an optical constitution of a stereo-microscope and more particularly to an optical constitution of an ophthalmological slit lamp stereo-microscope. 
The optical type of a stereo-microscope is largely classified into the Greenough type or the Galileo type.  The Greenough type is, as shown in FIG.  1, constituted as such that two optical paths I,II are crossed on a plane of an object to be observed at an included angle .omega.. The optical paths I,II have objective lens systems 1a, 1b, erecting optical systems 2a, 2b and ocular systems 3a, 3b, respectively, and are constituted as such that an incident optical axis IA toward the objective lens is parallel to an observation optical path IB of the ocular (likewise, in the optical path II, the incident optical axis IIA is parallel to the observation optical axis IIB).  In this case, the included angle .omega.  is set to be a convergent angle somewhere between from 10.degree.  to 16.degree.  which is very close to natural sighting, when an observer's naked eyes e.sub.1, e.sub.2 observe the object E from a near distance without using a microscope.  Therefore, the Greenough type microscope has such an advantage as that a natural stereoscopic sighting can be obtained, since the included angle .omega. ( hereinafter referred to as a stereo angle) defined between the incident optical axes IA, IIA toward the objective lens is equal to the included angle .theta. ( hereinafter referred to as an observation angle) defined between the observation optical axes IB, IIB of the ocular systems 3a, 3b and the stereo angle .omega.  is formed to be equal to the convergent angle of a natural sighting.  However, it had such disadvantages that since the optical paths I, II are crossed, machining for mounting optical parts are complicated, and the constitution of the focussing and variable power optical systems (not shown) which are normally arranged between the objective lens system 1a, (1b) and the erecting optical system 2a, (2b) are also complicated. 
On the other hand, the Galileo type stereo-microscope is, as shown in FIG.  2, constituted as such that the optical paths are parallel to the optical axis, respectively.  The optical path III comprises an objective lens system 4, an imaging lens system 7a for producing an intermediate image Pa and an ocular system 6a for observing the intermediate image Pa. The optical path IV has the objective lens system 4 as a common objective lens and comprises an imaging lens system 7b, an erecting optical system 5b and an ocular system 6b same as in the case mentioned with regard to the optical path III.  The optical axes of the both imaging systems 7a, 7b are parallel with respect to each other and also parallel to the optical axis 4a of the objective lens system 4.  The observation optical axes IIIB, IVB of the ocular systems 6a, 6a are likewise , constituted to be parallel with respect to the axes of the imaging lens systems 7a, 7b.  The stereo angle .omega.  defined between the incident optical axes IIIA, IVA toward the objective lens system 4 is decided depending on the base length l which is defined by the distance between the imaging lens systems 7a, 7b.  Because of the parallax due to this stereo angle .omega.,  the object E can be observed stereoscopically, although the observation angle .theta.  defined between the observation axes IIIB, IVB is 0.  The Galileo type stereo-microscope had such advantages as that since the two optical paths III, IV are parallel with respect to each other as mentioned above, the constitution of the optical system is simple, the constitution of the focussing structure and variable power structure is simple, and attached optical paths such as, for example, a camera optical system and a side observation scope can be comparatively easily added. 
Generally speaking, human eye does not have a convergence between the lines of sight of the both eyes when it is in the far distance sighting state, and can be maintained in comfortable observation state with less fatigue and without accomodation of the crystalline lens.  This is also true with respect to a microscopic observation.  It is said that the Galileo type microscope in which the both observation optical axes IIIB, IVB are arranged to be parallel with respect to each other is advantageous when used for a long time observation compared with the Greenough type microscope in which the both axes are convergent with respect to each other since the former is less fatiguing during observation. 
However, a microscope is a device for enlarging and observing a tiny object placed in a near position.  In the Galileo type microscope, although the luminous flux is entered into the eye as if it were entered from an infinite far distance, optically, the human brain has a preliminary information that it is looking a near object.  Therefore, it had such a disadvantage as that there is a gap between the natural sense of sighting and the optical sense of sighting.  Particularly, due to the information of the convolution motion, or due to the observation of parallax caused by the stereo angle .omega.  without the presence of the convergence motion information, the observer had an unnatural stereoscopic sighting stronger than normal stereoscopic sense. 
Furthermore, in the case of, for example, a stereo-microscope of a slit lamp which is used in the ophthalmological field, a doctor is often required to observe the eyes to be observed with the naked eye in order to adjust, for example, the illumination position, the width and length of the slit, or to conduct a simple operational treatment, the doctor's observation eye is put in a near range sighting state.  Then, the doctor has to look through the microscope and diverges the eye so that it becomes in a far range sighting state for observing the object stereoscopically.  However, there was such disadvantage as that instant image separation and stereoscopic sighting were difficult to be effected. 
The exaggeration of a stereoscopic sense and the necessity of effecting the divergence motion from the naked eye sighting to the stereoscopic sighting deny the littleness of the observation fatigue hitherto advocated with regard to the Galileo type stereo-microscope.  On the contrary, the observation fatigue is found to be greater than the Greenough type microscope.  In addition, there found to be such disadvantages as, for example, incorrect observation, or mis-treatment due to erroneous recognition of the actual distance.  All these disadvantages with respect to the Galileo type microscope are remained yet to be solved in spite of the afore-mentioned advantages it has. 
On the other hand, in the Greenough type microscope, although it had such advantage as that the object can be observed in the same stereoscopic sighting as that with respect to the near sighting with the naked eye even under the microscopic observation as mentioned above, when it is required to observe a very small unevenness, there are instances where it is difficult to observe such unevenness with the stereoscopic sense which is close enough to the natural sighting.  Therefore, there was a request to strengthen the stereoscopic sense with regard to the Greenough type microscope.