290 M. A. AINSLIE ON A VARIATION OF CHESHIRE'S APERTOMETER. 



we change the refractive index from 1 to ju, we merely have to 

 close up the radial lines in that ratio, leaving the curve unaltered. 

 For instance, if we had 16 radii for the dry form we could use 

 the same curve, but with 24 radii, for a plate of glass of 

 fx = 15. 



In practice the instrument proves of great utility, and very 

 reliable and easily used. All that is necessary is to be accurate 

 in centring ; this is easily seen to be correct when the reading of 

 each end of the spiral is the same. 





Journ. Quekett Microscopical Club, Ser. 2, Vol. XII., No. 74, April 1914. 



