468 



Mr. T. H. Littlewood on a Method of 



The liquid whose refractive index is to be determined is 

 placed in a glass vessel about 3 feet away from the telescope, 

 and with its level slightly below that of the telescope. 



A scale of glass (or some material not acted on by the 

 liquid) is placed vertically in the liquid and illuminated by 

 monochromatic light. The position of the telescope on the 

 horizontal scale is then taken when observing the various 

 divisions on the vertical scale. From these readings the 

 index of refraction can be ascertained. 



First case. — When the liquid is homogeneous (fig. 1) . 



Fig. 1. 





et ' & 



c a. 



A 



/ / / 

 '- /N / •' 



B 



IV / 



/ / 



/ 

 / 





* 





C 



* 

 * 

 » 



/ 



f 

 / 



/ 



r/ 





D 



■' 





Suppose A, B are two points on the vertical scale out of the 

 liquid ; C, D two points within the liquid. Suppose a, b y c, d 

 are the positions of the telescope on the horizontal scale when 

 observing these points. Then Aa, B6 are parallel ; and if 

 CMc, DM.'d are the directions of the axis of the pencil of 

 light from C and D, Mc, Wd are parallel, and also CM, 

 DM' are parallel. Draw AN and QW parallel to abed. 

 AN = ab 3 CW = cd. 



/. tanAB6=-||, and tan CDM'= ~ 



sin i _ sin ABb ah 

 ^ = slnT " sin CDM' ~ Vd X 



a / cc[i 



+ OD 2 



-f AB< 



