﻿88 & P. Langley — Unrecognized Wave-lengths. 



ible also, that lies the wave-length we are seeking ; conse- 

 quently, 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 D 2 , which pass simultaneously through 

 the slit S 2 . 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 60° 00' 00" and a temperature of 

 20° C.) of 41° 05' 40". Now on replacing the telescope by the 

 bolometer, the bolometer wire will feel this same ray which the 

 eye has just recognized hy its light, and, if the galvanometer 

 be in a sensitive condition, the image will be thrown by the 

 heat off the scale, while a little on either side of this position 

 no indication will be given. The beam and the slit S 2 remain- 

 ing in the same position, let us next suppose that the bolometer 

 arm is carried toward b, in the direction of B. There will be 

 no sensible deflection until it reaches the position b in the red, 



corresponding to a wave-length of O^'TOGS, and in the prism 

 to an angle of 40° 33 / nearly, for there is no sensible heat 

 except in the successive images of slit S 2 formed by the 

 prism P in the line PB. Passing farther toward B we come 

 into the heat in c, and next to the heat in d which is less 

 than -j-J-g- that in the direct prismatic image, when no grating 

 is employed. 



This was the utmost limit of our power of measurement in 

 1883, beyond this point radiations from the grating being then 

 absolutely insensible, and the radiation at the point d itself be- 

 ing excessively minute, even in the solar spectrum, where the 

 heat, so far as &ny is found, is as a rule far greater than that in 

 the spectrum of the arc. Accordingly I have elsewhere ob- 

 served that these measures could be carried on as well by a 

 large electric arc as by the sun ; but in fact, owing to the diffi- 

 culties attendant on bringing the arc, which must be of im- 

 mense heat, close to slit S 1} and to other causes, the sunlight 

 would be preferable wherever it could be used. 



Oar observation of June 7, 1882, gave the value of the index 

 of refraction corresponding to ^=2 ,u, 356, which was the lowest 

 possibly attainable by our then apparatus. Incessant practice 

 and study, resulting in improvements already referred to, have 

 enabled us finally to measure down to a wave-length of 9x£D 2 

 corresponding to a position much below/. We may add that 

 in doing so, it is sometimes convenient to employ a bolometer 

 wide enough to overlap the images in the other adjacent spectra 

 of the higher orders, which we may usually do without con- 



