Method of Measuring Refractive Indices. By E. M. Nelson. 657 
In tlie second the plano-concave is filled np with the medium, which 
is then covered by the plano-convex lens, but in the fourth and sixth 
it is covered bv the slip, and in the eighth by the plano-convex 
lens. It is obvious that in the even numbers (B, D, F, II), the 
medium acts as a convex, and in the odd (A, C, E, G) as a con- 
cave lens The method of measuring the locus is very simple: the 
medium having been put in position, the slide is first placed on the 
wooden slip and then on the top of the substage of the Microscope, 
which is used vertically. (The object of the wooden slip is threefold ; 
first to prevent the slide when heated touching the brass-work, 
secondly to act as a diaphragm, and thirdly to hold the lenses in 
cases E and G.) A slip with a mark on its lower surface, such as an 
ordinary stage micrometer, cover downwards, is placed on the stage, 
an inch or 2/3 objective is screwed on the nose-piece, and the mark is 
brought into focus. 
The substage is now moved until the image of a distant tree or 
chimney-pot is focused by the lenses containing the medium on the 
same mark. The plane mirror is of course used, and further it is as 
well to test the plane mirror by the sun’s rays, because so-called 
plane mirrors are often concaves of long foci, in which case they 
would be unsuitable for this purpose. 
The focus, F, is the distance measured between the medium and 
the mark on the lower side of the slip on the stage. 
With regard to the construction of the lenses, care should be ex- 
ercised to make them not only true to the radius of 1 in., but 
also of glass having precisely the same refractive index, which should 
be as close to 1*5 as possible. In the formulae /x is the refractive 
index of the medium to be measured, and f that of the glass of 
which the lenses are composed, F being the focus of the entire com- 
bination with the medium in position. 
Example : — In C let f = § , arid F = 2. 
Then ya = 3— 1 — J = f. 
In D let F = 2. 
Then /x = § + J = 2. 
My subject is now finished, but before closing it may interest those 
who study optics to know that these eight groups of lenses are not 
a haphazard arrangement. A little consideration will show that a 
single plano-convex lens of 1 in. radius arranged as in A will yield a 
focus of 2 to 4 in., with a variation in /x of 1 to 1 * 25, and it is 
equally easy to see that a focus of 4 to 2 in. will be obtained with 
a variation in /x of 1 * 25 to 1*5 by a plano-convex lens of the same 
radius composed of the medium itself. But as the medium must be 
