682 SUMMARY OF CURRENT RESEARCHES RELATING TO 



seen under any amplification — at most it could only cause some 

 difficulty by its uncouth perspective — so long as the visual impres- 

 sions furnished sufficient salient points for the construction of the 

 retinal image as a solid or three dimensional one. For this, however, 

 to be possible it is plainly essential that the solid object should be 

 simultaneously visible within a certain not too small depth ; for 

 obviously no indications could be derived of the constituents of space 

 if the Microscope at each adjustment only allowed a single layer of 

 inappreciable depth to be clearly recognized. All optical apparatus 

 for obtaining such indications with binocular vision must remain 

 ineffective if the images themselves give no clear impression of any- 

 thing that relates to the third dimension. 



The over-amplification of the dimension of depth inseparable from 

 the action of optical instruments comes, it will be seen, to be an 

 insuperable obstacle to efficient stereoscopic vision with high powers, 

 because as the consequence of that want of proportion in the solid 

 image — and for that reason only — the visual space of the Microscope 

 loses more and more in depth as the amplification increases, and 

 approaches more and more to a bare transverse section of the object. 



This visual space, that is the solid space occupied by the object, 

 which at one adjustment of the Microscope is plainly visible to the 

 eye, is made up of two parts, tlie limits of which as regards the fZepffe 

 are determined in a very different manner. 



First, the accommodation of the eye embraces a certain depth, 

 different planes being successively depicted with perfect sharpness 

 of image on the retma, whilst the eye — accommodating itself 

 consciously or unconsciously — is adjusted by degrees to virtual 

 images of greater and less distance of vision. This depth of accom- 

 modation, which of course in the perception of the relations of 

 space plays precisely the same part in microscopical as in ordinary 

 vision, is completely determined by the extent of accommodation 

 of the particular eye, the limits being the greatest and shortest 

 distance of distinct vision. Its exact numerical measure is the differ- 

 ence between the reciprocal values of these two extreme distances. 

 If the capacity of accommodation of a particular eye is exjiressed thus 

 numerically, the depth of accommodation for the same eye in micro- 

 scopical vision for any given linear amplification may be exactly 

 computed independently of the composition of the Microscope, 

 provided the index of refraction is given of the medium in wliich the 

 object under observation is placed. The depth of perfectly distinct 

 vision is directly proportional to the above-mentioned numerical equi- 

 valent of the extent of accommodation of the eye, directly proportional 

 to the refractive index of the medium of the object, and inversely pro- 

 portional to the square of the amjilification when referred always 

 to the same image-distance (say 250 mm.). Assume, for example, 

 that for a moderately short-sighted eye the nearest point of dis- 

 tinct vision is 150 mm., and the farthest point 300 mm. — in which 

 case the numerical equivalent of the extent of accommodation would 

 equal ^-J ^ mm. — the calculation for an object in air would give a depth 

 of vision by accommodation amounting to 



