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observer looks through two lenses at two photographs of 
some solid body, or of some landscape taken from slightly 
different points of view, and thus the same impression is pro- 
duced upon the brain as when the real object is viewed by 
both eyes. The microscopist who uses a binocular instrument 
sees one magnified image of an object, the rays proceeding 
from which are so refracted by the intervening prism as to 
enter both eyes as if direct from the image. It is evident, 
then, that these two instruments are not similar, and that 
any theory founded upon their similarity is false. The one 
image formed by an objective cannot by being separated into 
two acquire new relations to the figure of the object. It is 
true that what is called a stereoscopic effect is to a certain 
extent produced by the binocular instrument. But if two 
common cartes de visite of the same person are placed in a 
stereoscope, a stereoscopic effect will be produced, and yet the 
conditions required for forming the pictures correctly, namely, 
that they should be taken from two different points of view, 
have not been satisfied, and it therefore follows that the im- 
pression produced upon the brain is not accurately true. This 
case is somewhat similar to that of an image of a microscopic 
object viewed by both eyes at the same time with the aid of 
a prism. It is quite possible that the effect of the binocular 
would be to exaggerate the deviations in altitude on the sur- 
face of an object. Dr. Donkin’s remarks in his late paper 
go to establish the truth of these views. He says, ‘ Mic. 
Journ.,’ New Series, No. xxxv, p. 289: — “Mr. Ralfs, 
too, whose accuracy as an observer is sufficiently well 
known, says of the genus Hyalodiscus, ‘ Its flat disc will 
distinguish it from Podosira.’ Now, the fact is, that the 
valve of Hyalodiscus subtilis is hemispherical approaching 
indeed to conical, not a flat disc, as it appears to be under the 
monocular.” Now, if this were the case, it must have been 
detected by the changes of focus necessary to bring the 
different lamina into distinct vision when using lenses of 
high power and large angular aperture. Dr. Donkin does 
not appear to have tried this, but merely to have taken for- 
granted that the binocular must be right, and the monocular 
wrong. As the effect of the former depends on a kind of 
illusion, some proof is necessary that the illusion may not 
occasionally lead to delusion. Dr. Donkin’s name is well 
known in connection with Diatomacere, and he may very 
possibly be adequate to the arduous task which he has under- 
taken of issuing a new classification of these interesting 
forms, but it is to be hoped that he will not be guided in his 
work by observations taken with a binocular instrument. — 
F. G. Stokes, M.A. 
