228 



THE AMEKICAN MONTHLY [October, 



ground glass which admits light, and V is a screw to fix the 

 tube CD when it is so adjusted that R is at the focus of the lens. 

 To find the refractive index of a liquid, a drop is placed upon 

 the plane surface of L ; of the rays refracted through L, those 

 W'hich have an angle of incidence greater than the critical angle 

 are totally reflected at the surface of the liquid and illuminate 

 the lower portion of the reticule ; the upper part i-emains dark, 

 and the position of the boundary line depends upon the refractive 

 index. If the value of the graduations is known the index is 

 read directly from the position of this line upon the scale. 



Fig. 3. 



Fig. 4. 



For solids, a polished plane surface is placed against the lens, 

 a liquid of higher refractive index having been interposed between 

 them ; two boundary lines are then seen, one of which belongs 

 to the liquid and the other to the solid ; the latter gives the re- 

 quired index directly. 



The instrument is graduated by determining the position of the 

 boundary line for different solids and liquids of known refrac- 

 tive index.* 



The following is a very simple method proposed by Mr. Gor- 

 don Thompson, and which, it is claimed, yields sufficiently ac- 

 curate results for the ordinary purposes of the microscopist, 



A fine mark is made on an ordinary glass slide with a writing 



'Taken, in part, from Jour. Royal Micro. Soc, vol. 7, p. 469. 



