THE ELEMENTS OF MICROSCOPY. 45 
Into the jar, around which it is advisable to put a sheet of cotton 
wadding to protect it from injury, I generally put about a quart of 
the best methylated spirit, and in this the specimens may be kept 
for some time, but the sooner they are separated the better. Small 
and fragile specimens are best kept in small bottles, but not more 
than two or three should be put into one bottle. All specimens 
intended for microscopical work should be preserved in absolute 
alcohol, as by far the best results are obtained from its use. A one 
or two per cent. solution of potassium bichromate is also useful. 
Glycerine has been highly recommended for univalve molluscs, 
when the odontophores only are required. 
LEE \ELEMENTS «OF «MICROSCOPY, 
By GeorcE E. Davis, F.R.M.S.* 
TE 
Some of the properties of plates, prisms and lenses. 
@. 
N my last paper I pointed out to you some of the defects of the 
human eye, and showed how microscopical vision was affected 
by them. Your attention was also called to the fact that objects 
though seen plainly at certain definite distances, become indistinct 
and finally invisible by continued approach to the eye. The cause 
of this was also mentioned,—the rays proceeding from the object 
diverged too strongly, and thus the posterior focus is thrown behind 
the retina. A convex lens, if now introduced between the object 
and the eye; will render the object clearly visible, for the simple 
reason that the rays are more restored towards parallelism by means 
of the lens, and so form a posterior focus short enough to enable 
the picture to be thrown upon the retina. But, although the object 
previously invisible is now brought into view, yet it will appear 
larger than it really is, as the image presents itself to our visual 
organs under a larger visual angle. 
This brings me to the first point of interest in connection with 
practical optics, that is to say, with the subject of refraction. With- 
out the phenomenon of refraction glass lenses would be of but 
little service to us as magnifiers, and we should have to fall back 
upon reflectors as Amici did in the earlier years of the present 
century ; and although refraction is a very useful property in 
* A paper read before the Manchester Microscopical Society. The diagrams 
to which references will be found were shown as Lantern Transparencies. We 
regret that owing to their number they cannot be introduced into the text.—Ep. 
