ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
141 
that the mean of all the measurements using the flat cover is sensibly 
the same as with the other covers (some of which possessed a curvature 
quite sensible), the difference being only 0 • 02 /x, which is too small to 
measure. 
It was intended to make a much larger scries of measurements, but 
the pressure of other work prevented. 
The foregoing are, however, sufficient to warrant the conclusion (to 
which there may possibly, in the case of very bad covers, be some 
exceptions) that using good cover-glass, such as is furnished by 
reputable dealers in this country, the most, if not all, of which is 
manufactured by Chance, the influence of the curvature of such covers is 
practically nil. 
This is not the conclusion to which the writer had come from a 
priori reasoning ; but it is the only one warranted by such facts as have 
been observed. Further observations will be presented in a future paper.” 
Simple Method of Finding the Refractive Index of various Mount- 
ing Media. — Mr. E. M. Nelson communicates the following : — Provido 
two precisely similar equi-convex lenses, whose identical refractive 
index /x and radii r are known, and cement them together with the 
mounting medium whose refractive index has to be determined. 
Now measure F the principal focus of the combination, then the re- 
fractive index of the mounting medium. 
It is convenient to make the radii of the equi-convex lenses 2 in. 
Then 
Some examples might be of interest. 
Let the refractive index /x of the two equi-convex lenses be 3/2, and 
suppose that the combination has no focus, that is, that it behaves like a 
piece of plane glass, then 
and 
p’ = 2/x — 1 = 2-0. 
If the principal focus of the combination F = -f- 2 then 
or the same as the equi-convex lenses. 
But if the principal focus of the combination F is negative, it must 
be measured in the same way as a concave spectacle lens, viz. by neu- 
tralizing it by a, positive lens of equal focus. If F is negative the sign 
before the fraction will be changed. 
Example, let 
F = — 2. 
/ = 2/4-1- -L = 2«-l+I= 2-5. 
— 2 1 2 
Then 
