336 | THE NORTHERN MICROSCOPIST. 

cope acquires more and more the property of an oftical microtome, 
which presents to the observer's eye, sections of the object of a fine- 
ness and sharpness that no instrument could produce by mechanical 
means.” 
Another novel point was the demonstration of the very material 
distinction between ordinary stereoscopic vision and that with the 
Microscope. ‘The perspective shortening of the lines and surfaces 
by oblique projection, which is an important element of solid vision 
with the naked eye, is wholly wanting in microscopical vision, in 
which we have only the other element, a relative displacement of 
successive layers in the image. That these displacements are seen 
in the Microscope, depends entirely on the peculiar exaggeration 
in the amplification of the depth of an object which is not found in 
ordinary vision. 
The paper “On the Conditions of Orthoscopic and Pseudoscopic 
Effects in the Binocular Microscope ” is also a most useful contribu- 
tion to the theory of micro-stereoscopic vision, establishing as it 
does the true criteria for both classes of effects, and at the same 
time clearing up a misconception that had arisen as to the supposed 
necessity for the rays from the two halves of the objective crossing 
in order to get proper orthoscopic effect. If the delineating pencils 
have been reflected an even number of times in the same plane, the 
rays must cross, but otherwise not. 
Mounting-Media of High Refractive Indices. 
To utilize the full benefit of immersion objectives, it is of course 
essential that the object should be mounted in a medium, the re- 
fractive index of which is not less than that of the immersion fluid ; 
and down to a comparatively recent period Canada balsam was 
most commonly used for this purpose, particularly for diatoms. 
Mr. Stephenson, however, pointed out that although by the use 
of the balsam we have attained our object so far as the aperture is 
concerned, yet we have done so at the expense of the visibility of 
the resultant image, which has become fainter by the nearer 
approximation to equality of the refractive indices of the diatoma- 
ceous silex and the balsam ; the visibility of minute structures being 
proportional to the difference between the refractive indices of the 
object and the medium in which it is mounted. Instead of balsam, 
therefore, media of high refractive index should be employed ; 
thus, as the refractive indices of diatomaceous silex and Canada 
balsam are respectively 1.43 and 1.54, the difference .11 is the 
measure of the visibility of a diatom in balsam. Using a solution 
of phosphorus in bisulphide of carbon, the refractive index of 
which is 2.10, the difference is .67, and the visibility of the diatoms 
is now more than six times as great as it was in the balsam. 
Continuing his researches on this subject, and endeavouring to 
