NO. 8 SAMUEL PIERPONT LANGLEY — ABBOT 2'] 



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 S2 remaining in the same position, let us next 

 suppose that the bolometer arm is carried toward h, in the direction of 

 B. There will be no sensible deflection until it reaches the position h 

 in the red, corresponding to a wave-length of o'*.7o68, and in the prism 

 to an angle of 40° 33' nearly, for there is no sensible heat except in 

 the successive images of slit So 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 i/ioo 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 elsewhere 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 Si, and to other causes, the sunlight 

 would be preferable wherever it could be used. 



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

 refraction corresponding to A = 2^^.356, which was the lowest possibly 

 attainable by our then apparatus. Incessant practice and study, result- 

 ing in improvements already referred to, have enabled us finally to 

 measure down to a wave-length of 9XAD0 corresponding to a posi- 

 tion 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 feeljleness compared with 

 that of the first spectrum in which we are searching. 



" We usually, however, employ a bolometer of not more than i mm 

 aperture, and this demands excessive delicacy in the heat-measuring 

 apparatus, since the heat here is, approximately speaking, about 

 i/ioooo of that in the region between the sodium lines in the direct 

 spectrum of a rock-salt prism. This is near the limit of our present 

 measuring powers with the grating, even when every possible device 

 is used to increase the extremely feeble heat in this part of the 

 spectrum. 



