The Three Secretaries 219 



While the visible or photographic spectrum includes rays of 

 only about an octave of vibration between the waves of 

 violet and red, the full spectrum, from the ultra-violet rays 

 to the longest of those measured by the bolometer, embraces 

 between five and six octaves, and still more are indicated. 

 In one sense these investigations have partly bridged over 

 the gulf between the longest wave-length of heat and the 

 shortest waves due to other causes. "This work," says 

 Lockyer, " has done for the lower spectrum what that of 

 Kirchhoff did for the upper rays." 



Father J. Van Geersdale, of Louvain, in an article on "The 

 Infra-red Spectrum and the Bolometer," written in 1896, 

 remarks : 



" Newton would be very greatly surprised if, coming back 

 for a moment to this world, he should have placed before 

 him a map of the spectrum as it is known to-day. Not only 

 would he be astonished at the numberless rays which were 

 unknown to him, but he would be still more taken aback if 

 he saw the spectral image lengthened until it had assumed 

 dimensions fifteen and twenty times as great as those which 

 he gave to it. In his day, below the violet (X= 0,42), and 

 above the red (X = 0,67), there was absolutely nothing. To- 

 day the researches of Cornu, Mascart, Schumann, and others 

 have expanded the limits of the ultra-violet to the neighbor- 

 hood of A =0,1. In the other direction, the investigations 

 undertaken by Mr. Langley in the infra-red region have 

 resulted in an acquaintance with bands and rays the wave- 

 length of which reaches to six microns and beyond. 



" Without depreciating the value of the researches which 

 were made in the less refrangible portions of the spectrum 

 previous to the discovery of the bolometer, it must be ad- 

 mitted that they were of very slight moment if we now com- 

 pare them with those which Mr. Langley has obtained by the 

 aid of his marvelous little instrument." ^ 



l"Le Spectre Infra- Rouge et le Bolometre," Revue des Questions Scientifiques, 

 Volume X, page 26, July, 1896, Louvain. 



