REFRACTIVE INDEX. 



121 



Pe Chaulnes' Method for Determining Refractive 



Index. 



If light be converged by a lens L 

 and passed through a glass slip or 

 mineral section with parallel faces, 

 each ray is refracted. Suppose 

 the converged light after passing 

 through the shp AB to be received 

 into a microscope which has OB 

 the normal to the plane faces of 

 the slip as its line of collimation. 



Let PBQ be the angle that the 

 converged light makes with the 

 normal to the plane faces of the 

 slip. 



On entering the sUp the ray will 

 be deflected along some direction 

 BD such that sin. PBO : sin. ABD 

 =/< : I where /» is the refractive 

 index of the substance of which 

 the slip is made. 



On emerging from the shp the 

 light is again refracted and passes 

 along a path DX which is parallel 

 l"iG. 2. to its former direction PB, 



If an object lies at B,then the introduction of the slip causes 

 it to appear at F. 



If the microscope was focussed on the object at B before the 

 introduction of the slip, then in order that the object may be in 

 focus after the introduction of the shp it will be found necessary to 

 raise the objective through a distance FB. 



The refractive index of the shp can be expressed in terms of 

 the distance BF and the thickness of the mineral section as follows : — 

 Let BF=daindAB=t, 



Then the refractive index is given by the relation 

 sin. PBQ _ tan. PBQ 

 ^'~sin. AbB ~ taiTABD '^^^^^ ^^^ ^"gles PBQ and 



ABD are small. 



Hence "=^§~| = AB/AF=^(/-rf) 



(1) 



To investigate the limits of error the following procedure is 

 adopted. 



Let /n denote tlie refractive index 



Then « =tl{t^d) 



