300 



INFRA-RED EMISSION SPECTRA. 



also serves as the vane of a radiometer, suggests itself. Even the finest 

 quartz fibers are quite strong, so that they would support the extra 

 weight of the loop of conducting wire which passes between the poles 

 of the magnet. The extra weight would increase the period, while a 



coarser fiber would decrease it ; but whether the selection of the proper 

 combination would compensate for the new difificulties involved remains 

 to be determined. 



OTHER EXPERIMENTAL DETAILS. 



For the adjustment and calibration of the apparatus, the reader is 

 referred to the memoir on "Absorption Spectra" (loc. cit.), and only 

 certain details in the manipulation of the experiment will be mentioned. 



Since the arc varies with such great rapidity it was necessary to have 

 a radiometer of quick period, as already mentioned, while special pro- 

 vision had to be made to manipulate the arc and observe the deflections. 

 The length of the arc was regulated by securing the electrodes in 

 holders which could be separated or closed by means of a rack and 

 pinion. This was mounted upon a holder which was in an asbestus- 

 lined box, having a heavy sheet-iron shield through which the energy 

 from the arc passed into the radiometer slit. The slit in the shield 

 was I cm. high, while the length of the arc, which was directly back of 



