A Simple Method of Finding the Refractive Index of 

 Various Mounting Media. 



. By E. M. Nelson, F.R.M.S. 



(Taken as read, January 15fh, 1892.) 



Provide two precisely similar equi-convex lenses, -wliose 

 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 refractive index of 

 the mounting medium. 



^ ^ 2F 



It is convenient to make the radii of the equi-convex lenses 

 two inches. Then — 



^, = 2^-1-1 



Some examples might be of interest. 



Let the refractive index /x of the two equi-convex lenses be 

 f, and suppose that the combination has no focus, that is, that 



it behaves like a piece of plane glass, then F = oo,— = 0, and 

 /x'== 2/A— 1 = 2-0. 



If the principal focus of the combination F = -f 2 then 



/x.' = 2 ju. — 1^ := _ 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 neutralizing it by a positive lens of equal focus. 



If F is negative the sign before the fraction will be changed. 

 Example, let F = — 2. Then — 



/.' = 2/.-l— _i = 2yu~l-fi = 2-5. 



The above method gives a greater range of readings for 

 indices varying from 2*0 to 2'5, and consequently more accurate 

 results than the simpler one of filling up a plano-concave lens 



