88 VISION WITH THE COMPOUND MICROSCOPE 



mat ter was involved in obscurity. The remarkable insight, and 

 learning of Professor Abbe have, however, found for this important 

 subject a sound scientific basis. 



The delineation of solid objects by a system of lenses is by 

 virtue of the most general laws of optical delineation, subject to a 

 peculiar disproportion in amplification. The linear amplification of 

 the depth-dimenxum is, when both the object and the image are in 

 the same medium (air), found to be always equal to the square of 

 the linear amplification of the dimensions at right angles to the 

 optical axis ; but if the object be in a more highly refracting medium 

 than air, it is equal to this square divided by the refractive index 

 of the medium. In proportion to the lateral amplification there is 

 a progressive, and with high powers a rapidly increasing, over- 

 amplification of the depth of the three-dimensional image. If a 

 transverse section of an object is magnified 100 times in breadth the 

 distance between the planes of parts lying one behind the other is 

 magnified 10,000 times at the corresponding parts 011 the axis when 

 the object is in air, 7500 times when it is in water, and 6600 time- 

 when it is in Canada balsam. 



This excessive distortion in the case of high amplifications is not, 

 however, of itself so complete a hindrance to correct appreciation of 

 solid forms in the microscopical image as at first appears. The 

 appreciation of solid form is not a matter of sensation only ; it is a 

 mental act a conception and, therefore, the peculiarity of the 

 optical image, however great the amplification, would not prevent 

 the conception of the solidity of the object so long as salient points 

 for the construction of a three-dimensional image w r ere found. But 

 for this the solid object, as such, must be simultaneously visible ; 

 a single layer of inappreciable depth can convey no conception of 

 the three space dimensions possessed by the object. 



Owing to the disproportion! amplification of the depth-dimension 

 normal to the action of optical instruments, the visual space of the 

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

 and thus constantly approximates to a bare horizontal section of the 

 object. 



The visual space, which at one adjustment of the focus is plainly 

 visible, is made up of two parts, the limits of which as regards the 

 depth are determined in a very different mariner. 



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

 different planes being successively depicted with perfect sharpness 

 of image on the retina, whilst the eye, adjusting itself by conscious 

 or unconscious accommodation, obtains virtual images of greater or 

 less distance of vision. This depth of accommodation, which pla.vs 

 the same part in microscopical as in ordinary vision, is wholly 

 determined by the extent of power in this direction possessed by the 

 particular eye, the limits being the greatest and the least distance 

 of distinct vision. Its exact numerical measure is the difference 

 between the reciprocal values of these two extreme distances. The 

 depth of distinct vision is directly proportional to this numerical 

 equivalent of the accommodation of the eye, directly proportional 

 to the refractive medium of the object, and inversely proportional 

 to the square of the amplification when referred always to the same 



