ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
875 
paper is placed under the ocular, and the room darkened, when a 
brilliant image will be apparent on the drawing paper. It is evident 
that in this way the artist has the advantage of perfect freedom, both 
of eyes and hands, and can trace the minutest detail with ease and 
accuracy. 
The Microscope and a Hair.* — Two different men were suspected 
of making an assault, but no proofs were forthcoming. A single hair 
which was found on the clothes of the victim finally became the clue 
to the mystery. 
The hair was photomicrographed and compared with photomicro- 
graphs of the beard and hair of each suspect. There was entire lack 
of similarity and the case was about to be abandoned. The hair was 
pointed and had never been cut. Other facts pointed to its belonging to 
a smooth-haired and comparatively short-haired dog. Inquiry revealed 
the fact that one of the suspects owned such a dog. A fresh hair 
agreed in every respect with the specimen. The owner of the dog 
could not explain away the facts, and was convicted. He confessed 
indeed to having committed the assault. 
(5) Microscopical Optics and Manipulation. 
Simple Method of Finding the Refractive Index of various 
Mounting Media.f — Mr. E. M. Nelson suggests the following method. 
“ Provide two precisely similar equi-convex lenses, whose identical refrac- 
tive 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 refractive 
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 
F = oo , ^ = 0, and u! = 2 ju — 1 = 2*0- 
Ju 
If the principal focus of the combination F = + 2, then 
/x' = 2/x - 1J = 3/2, 
or the same as that of 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. 
* Microscope, xii. (1892) p. 176. 
t Journ. Quek. Micr. Club, v. (1892) pp. 8-9 (with an addition by Mr. Nelson). 
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