124 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 



It is also to be borne in mind that by using very con- 

 densed sunlight, or by resorting to fluorescent or other 

 optical contrivances, as several experimenters have done, 

 the range of vision may be carried beyond the proper 

 violet limit. 



The principles here indicated must not be restricted 

 to the luminous radiations; they apply to all others too. 

 Thus if a photographic sensitive surface be made to re- 

 ceive the first spectrum, it will be impressed by certain 

 of its radiations, chiefly by those above the line Gr. If 

 it be exposed in the second, third, etc., spectra, it will 

 again be impressed by the corresponding undulations, 

 having two, three, etc., times the former length. From 

 this it may, therefore, be inferred that a chemical decom- 

 position of a given substance, brought about by undula- 

 tions of a certain length, will also be accomplished by 

 radiations that are octaves of the first. 



It has been stated that a dark space,j9, intervenes be- 

 tween the violet end of the first spectrum and the bright 

 streak A'. This dark space is at present an attractive 

 and wonderful field of optical investigation. 



A' 



Fig. 12. 



In Fig. 12, let A' represent the white streak in the 

 position of A' in Fig. 11 ; then from A' to v is the first 

 dark space,^) ; from v to r, the spectrum of the first order, 

 its violet end, v, nearest to A', its red end, r, more dis- 

 tant; from r to v' the second dark space; and from v' 

 to r' the spectrum of the second order. The third spec- 

 trum overlaps this second, and the fourth the third, etc. ; 



