Method of Measuring Refractive Indices. By E. M. Nelson. 659 
held in a concave lens, it is necessary to neutralize this concave lens 
by a plano-convex of similar glass and radius, as in B. 
It is not so easy, however, to discover a law for the arrangement 
of the next six. The method is explained thus : — 
When fi = § an increase of 0 1 5 is an increase of The 
O 
difference in the focus F caused by this increase may be determined 
thus : — 
F r r 3 f 
So when fi = |, F = 2 r, or the focus of a plano-convex lens. There- 
fore to maintain the same focus, a plano-convex lens must be added to 
A to obtain 0, and another to get E, and so on with the odd numbers. 
With the even numbers we have to add a plano-concave ; thus D is 
obtained from B, because taking away the plano-convex lens is tanta- 
mount to adding a plano-concave, and by adding another concave to 
D we obtain F, and so on. 
