OF ARTS AND SCIENCES. 513 



for the invisible ones their deviation was measured, and therefore 

 their indices of refraction. Two results were reached: 1. The rela- 

 tion between the wave-length and the index of refraction for the 

 invisible rays was determined experimentally : 2. The validity of 

 the theoretical formulas of Cauchy, Redtenbacher, Briot, and Wullner 

 for expressing this relation was confronted with the facts of observa- 

 tion, in order to discover which of them were most trustworthy, and 

 within what limits the best could be safely used. 



In August, 1886, Professor Langley published a paper on " Hitherto 

 unrecoo^nized Wave-Lengths," in which he describes a modification in 

 the bolometer arrangement by which it was adapted to the electric light. 

 His latest refinements on the bolometer, supplemented by an improved 

 Thomson galvanometer for measuring its indications, have culminated 

 in an instrument capable of revealing the presence of one millionth of 

 a Centigrade degree, and of giving reliable measures down to the hun- 

 dred-thousandth of a degree. He gives additional observations on wave- 

 lengths, deviations, and indices of refraction, made with three different 

 gratings of Rowland, and with lenses and prisms of rock-salt in place 

 of glass. In applying the formulas for dispersion the constants must 

 be determined by observations with highly dispersive prisms. This 

 was first done by Professor Baden Powell of Oxford. Adopting 

 Cauchy's formula, he came to the conclusion that the greater wave- 

 lengths converged towards a constant value, and that the maximum 

 value was not far beyond the red. Professor Langley has compared 

 his experimental results with the various formulas which assign the 

 relation between wave-length and the index of refraction, and has 

 come to the conclusion that, while there is great choice between 

 them, no one of them can be trusted in its application to wave-lengths 

 which are far beyond the visible spectrum. 



This paper concludes with an interesting summary of the extreme 

 range in the length of heat-waves, as given by the eye alone, or by 

 the united aid of phosphorescence, photography, and the bolometer. 

 While Newton's scale of wave-lengths had a compass of less than an 

 octave, which others in recent times have extended, somewhat doubt- 

 fully, to about an octave and one half, the bolometer in the hands of 

 Professor Langley has ranged over eight times the length of Newton's 

 spectrum by actual measurement, and has indicated the existence of 

 energy in a field six times greater for some kind of heat. In one 

 sense Professor Langley has bridged over the gulf between the short- 

 est wave-length of sound and the longest wave-lengtli of heat. If, 

 however, he should succeed, as he certainly may, in discovering waves 



VOL XXII. (N. 8 XIV.) 33 



