﻿918 Prof. C. Barus on the 



grating. The constants of these curves will be computed 

 below and their character will then appear. 



6. Independent Measurements. — As an example of coor- 

 dinated work, in which the index of the liquid and of the 

 solid are simultaneously found, the following experiment 

 may be cited. The air plate was enclosed between two thin 

 plates of microscope cover-glass. In such a case the limits 

 of total reflexion are determinable both for the reflected rays 

 and for the transmitted rays. The angle <f> in the former 

 ca^e gives the index of refraction n of the glass in relation to 

 the liquid (carbon disulphide) since 



n = N sin (/> • 



whereas the angle <£ in the latter case gives the index of the 

 liquid alone, or 



N=l/sin<J> . 

 Hence 



ra=sin <£/sin <£ , 



needing no correction for temperature. 



In an experiment made at 18°*3 with the plate of micro- 

 scope glass in question enclosing the air film, = 68°*9O anil 

 </> =38°'02, whence n=l-519tf. Using <£ = 68°*90 as found 

 and the tabulated data for carbon disulphide from Kohlrausch, 

 N = 1*6291, ft = 1*5199, which happens to agree accurately 

 with the preceding n. 



7. Prism (Liquid CS 2 ) and Submerged Grating. — The 

 present method for the above purpose makes use of the 

 grating directly. This is enclosed in a suitable closed trough, 

 preferably triangular in form, with windows of plate glass 

 and filled with carbon disulphide. If the grating is mounted 

 with its ruled face coinciding with the axis of rotation, as in 

 Kohlrausch's total reflectometer, the angle of total reflexion 

 may be measured in all parts of the spectrum. The Fraunhofer 

 lines which are simultaneously in focus are used for reference 

 in succession. Parallel rays of sunlight from a collimator 

 are thrown into one face of the liquid prism, nearly normally 

 to its plane. They emerge at the other face. The undeviated 

 rays are ignored, while the telescope is directed toward the 

 first order of diffraction spectra, this emergence also, for the 

 given grating, being not far from normal. Like the first 

 order of spectra, the second and third orders may often be 

 used with advantage, the limit of total reflexion being iden- 

 tical in all spectra on the same side of the undeviated ray. 



The limit of total reflexion moves through the spectrum 

 both with the rotation of the prism as a whole (which must 



