154 Prof. S. P. Langley on 



perature 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 by 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 remaining in the same position, let us next 

 suppose that the bolometer-arm is carried towards 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 0^7068, 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 P B. 

 Passing further toward B we come into the heat in c, and next 

 to the heat in d, which is less than t Jq 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 

 being excessively minute, even in the solar spectrum, where 

 the heat, so far as any is found, is as a rule far greater than 

 that in the spectrum of the arc. Accordingly I have else- 

 where observed that these measures could be carried on as well 

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

 the difficulties attendant on bringing the arc, which must be 

 of immense heat, close to slit S 1? and to other causes, the sun- 

 light would be preferable wherever it could be used. 



Our observation of June 7, 1882, gave the value of the 

 index of refraction corresponding to \= 2^356, which was 

 the lowest possibly attainable by our then apparatus. Inces- 

 sant practise and study, resulting in improvements already 

 referred to, have enabled us finally to measure down to a 

 wave-length of 9 x \D 2 , corresponding to a position much 

 below/. We may add that in doing so, it is sometimes con- 

 venient 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 confusing them, owing to 

 their feebleness compared with that of the first spectrum in 

 which we are searching. 



We usually, however, employ a bolometer of not more than 

 1 millim. aperture, and this demands excessive delicacy in the 

 heat-measuring apparatus, since the heat here is, approxi- 

 mately speaking, about ^owd of that in the region between the 

 sodium-lines in the direct spectrum of a rock-salt prism. 



Errata. 

 Page 158, line 4, after amperes add through the coils of 20 ohms resistance. 



— 163, — 20, for 39 c 15' 7" read 39° 15''7, and so to end of column. 



— 168, — 8, for +2 read +Q. 



