26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



end — the opening up to research of the whole unexplored region of 

 infra-red energy, not only from celestial but from terrestrial sources — 

 which will, we trust, justify the labor devoted to the following 

 determinations." 



He describes the arrangement of his apparatus, which includes a 

 diffraction grating and a prism in tandem. A beam of radiation from 

 the sun or the electric arc first traverses the diffraction grating spectro- 

 scope, whereby a group of rays of even multiples of the wave length 

 of a certain selected visible ray are all concentrated upon the slit of 

 the prismatic spectroscope. In the latter, the prismatic deviations are 

 measured, and from them are readily computed the indices of refrac- 

 tion of each of these rays of selected wave lengths. 



" There are in fact, passing through the same slit and lying super- 

 posed on one another by an unavoidable property of the grating, an 

 infinite number of spectra in theory, of which in this case nearly 

 twenty are actually recognizable, by photography, by the eye, or by 

 the bolometer, and of which, to consider only those where the wave 

 length is equal to or greater than that of the sodium line D.' we have 

 six spectra as follows : 



Wave length 



a. (visible) 6th spectrum D2 X = 0/^.5890 



b. " 5th " 6/5 D2 0.7068 



r. (mvisible) 4th " 6/4 Da 0.8835 



d. " 3d " 6/3 Da I .1780 



e. " 2d " 6/2 D2 1.7670 



/. " 1st " 6 D2 3 -5341 



" It is in this invisible underlying first spectrum, buried, so to 

 speak, beneath five others, of which three are themselves invisible 

 also, that lies the wave-length we are seeking ; consequently, there are 

 (to consider no others) at least six qualities of heat, of six distinct 

 refrangibilities, whose wave-lengths are equal to or greater than that 

 of D2, which pass simultaneously through the slit So. They pass 

 through the prism, and on looking through a telescope occupying the 

 position of the bolometer tube, we shall by suitably directing the arm 

 of the spectroscope see the light from the sixth one at a. Its wave- 

 length will be 0^^.5890, corresponding to a measured deviation (in 

 the case of the rock-salt prism, of an angle of 6o°oo'oo" and a tempera- 

 ture of 20°C.) of 4i°o5'4o". Now on replacing the telescope by the 



* We have heretofore adopted Angstrom's notation in calhng the more 

 refrangible sodium line ' Di'. We shall hereafter, however, in conformity with 

 the now more general usage, call this line, whose wave-length in Angstrom is 

 5889, ' D2'. The corrections to Angstrom are due to the researches of Messrs. 

 Peirce and Rowland. 



