ON RECENT RESEARCHES IN THE INFRA-RED SPECTRUM. 407 



The processes which involve the use of phosphorescent substances have 

 given some indications of lines considerably below 1", but it is safe to 

 state that the work which has just been referred to as communicated to 

 this Association in 1882 presents almost the only indications which we 

 have possessed, even up to the present time, about the lower infra-red 

 solar spectrum. 



Now the curve which was given, even in the later Alleghany observa- 

 tions, made with the rock-salt prism, contained but a dozen inflections 

 below the wave-length of l^'S, and these inflections, with their correct 

 prismatic and wave-length positions, represent, I think, most of our present 

 knowledge in these regions, even to-day. 



To understand the method by which there were attained, but only at 

 this great cost of labour, results till then unreached, it may be repeated 

 that the bolometer had been rendered more sensitive than the thermopile, 

 but that it was capable of being pointed, and its position in the spectrum 

 being measured only by a tedious process, which has been exclusively used 

 till lately (but which that presently to be described advantageously replaces). 

 Whichever process is used, when the bolometer thread touches a cold line 

 in the spectrum (since what is black to the eye is cold to it), a larger 

 current flows through the galvanometer, and the spot of light marking tlie 

 needle's motion is deflected through a certain number of degrees. 



From this point forward, the new process, whose results I am about to 

 have the pleasure of bringing before you, differs widely from the old. In 

 the old, two observers at least are engaged : one, who notes that reading 

 of the micrometer or of the vernier, which fixes in angular measure the 

 exact part of the spectral region whence (though nothing is visible) a 

 thermo-electric disturbance has proceeded ; and another, who simul- 

 taneously notes through how many divisions of the scale the spot of light 

 from the galvanometer mirror is deflected by the same electric disturbance. 

 The process may be compared to a gi-oping in the dark, and it was only by 

 these means that the considerable inflections of the energy curve much 

 below the region about 1" were then fixed by the bolometer, by being gone 

 over again and again with what seemed almost interminable repetition, 

 and which did in fact call for over a thousand galvanometer readings to 

 obtain the position and amount of each single inflection of the energv 

 curve, with the degree of accuracy which was then obtained, and whieli 

 was shown in the former memoir. 



If it took two years to fix the position of twenty lines by this process, 

 it would take two hundred years to fix two thousand, supposing they 

 existed, and it became evident that if the bolometer continued to be the 

 only means available, new and more effective methods of using it must be 

 found. 



New MetJwds. 



About ten years ago a plan was first studied, which has ever since been 

 maturing, by means of which this work could be carried on, not only with 

 far greater rapidity, but with greater certainty, and by an automatic 

 process. The idea in its original simplicity is very easily understood. 



In the old process, just described, the deflection of a spot of light upon 

 a scale was read by one observer, while another read simultaneously the 

 position in the spectrum of the cold band, or line, which caused the thermo- 

 electric disturbance. 



Now, in imagination, let us take away both the observer at the circle 



H H 2 



