METHOD OF FINDING THE REFRACTIVE INDEX OF GLASS. 273 



On a Method of Finding the Refractive Index of 

 Glass by Means of the Microscope and White Light. 

 By G. W. Royston-Pigott, M.A., M.D. Cantab., 

 F.C.P.S., M.R.C.P., F.R.A.S., M.R.I. 



The following method has occurred to me whilst writing 

 these papers : 



If d be the distance between an object (as micrometer lines, 

 placed on the stage) and the image formed, /the focal length 

 of the lens, and m the magnifying power at the distance 

 d, then (' Phil. Trans.,' p. 594, vol. ii, 1870) I have shown 



/ = 



m 



Suppose now, in order to obtain a very distinct image at a 

 distance from the object of 10 inches, a glass lens, fastened 

 to the nose of the microscope, be stopped off, so as to leave a 

 small central aperture, then, provided the curvature of the 

 lens is known (which, indeed, can be worked to any radius), 

 and the lens be very thin. 



r + s d 



or the refractive index, diminished by unity, is equal to the 

 products of two results, viz. : 



1. The product of the radii of the lens, divided by their 

 sum. 



2. The reciprocal of the focal length, or unity divided by 

 the focal length. 



If the lens be plano-convex, the first quantity is reduced 

 to half the radius : if the plane side be turned to the object. 



Example. — A crossed lens, having two radii respectively, 

 of 1 inch and 6 inches, is fixed on the microscope ; so that 

 an image of the micrometer is observed with a pocket lens 

 to be magnified (at a distance of 12 inches) exactly 6^ times, 

 a small central aperture being used : find the refractive index 



