of Infra- Red Rays in Rock Salt and Sylvine. 35 



and b, a concave mirror which concentrates the rays from a on 

 the slit s Q . Behind the slit s the rays traverse the sharp prism 

 j?, made of the substance to be examined, and fall on the con- 

 cave mirror e . By means of this arrangement a spectrum is 

 thrown in the plane of the slit of the spectrometer s 1 e 1 e^ s 2 , 

 out of which spectrum the slit s 1 cuts a narrow band, the mean 

 wave length of which can be varied at will by rotating the 

 mirror e about its vertical axis. On the table of the spec- 

 trometer s l e 1 e 2 s i a diffraction grating g of parallel silver wires 

 was mounted for the measuring of the wave lengths of the rays 

 passing through the slit s v The rays emerging from the fixed 

 ocular slit s 2 were concentrated by means of a concave mirror 

 of short focal distance on one wing of a Nichols radiometer.* 

 The window of the radiometer was a chloride of silver plate, 

 about l mm thick and the wings of the suspension were coated 

 with iron oxide. 



The spectrometer s } e x # 2 s 2 is the instrument whose construc- 

 tion and adjustment have been described at length in the 

 above mentioned paper,f and we can in consequence refer to 

 the description there given. However, it is necessary to enter 

 more fully into the theory of the spectral arrangement, con- 

 sisting of the slits s and s v the prism p and the concave mir- 

 ror e Q . Figure 2 represents the path of the middle ray when 



the deviation in the prism is (as a dotted line) and, when the 

 deviation is a (as a full line). In order that rays which are 

 deviated in the prism p through the angle a shall fall in the 

 slit s t it is necessary to turn the ray e through the angle 7, 



that is, the mirror e a through the angle — . But 7 = a— /3- and 



z 



approximately & : 7 = e p : ps — hence 7 = a(ps /s e ). The 

 fraction ps /s e can be easily determined by measurement to 



*E. F. Nichols, BerliDer Berichte, p. 1184; Wied. Ann., lx, p. 403, 1897 

 Physical Review, vol. iv, No. 22, p. 298, 1897. 



•j Rubens and Nichols, Wied. Ann., lx, p. 422, 1897. 



