22 INFRA-RED ABSORPTION SPECTRA. 



The vanes were suspended from a cross-bar by means of a very fine 

 quartz fiber, made by blowing it out in an oxyhydrogen flame. The 

 vanes were of very thin mica, approximately 15 by 2.5 mm., and were 

 held together by means of glass fibers, the distance between them being 

 about 4 mm. On the line midway between the vanes was fastened a 

 glass fiber, nm, which forms the axis of rotation, and carries near its 

 lower end a bit of mirror, made by silvering a thin microscope-glass 

 cover, which was then cut into areas of about 2 by 3 mm. The vanes 

 were blackened by burning camphor gum. To cause the lampblack to 

 adhere better to the mica a very thin coating of beeswax was first 

 applied by means of a hot wire. When the vanes were held above the 

 burning gum the wax softened and the soot was deposited in an even 

 layer. The vanes, mirror, and fiber were fastened by shellac. The 

 total weight of the suspension was about 10 mg. This was rather 

 heavy, but since no other precautions were taken against earth tremors, 

 it was more serviceable than a second one which was lighter. Gener- 

 ally there was no difficulty in reading to tenth millimeters on a scale 

 situated 1.4 meters from the radiometer. 



The sensitiveness of the instrument depends very much upon the 

 nearness of the vanes to the inner rock-salt window. This distance 

 was regulated by the tripod screw, which was on the rear side of the 

 radiometer base. The pressure for maximum sensitiveness, measured 

 with a McLeod gage, was from 0.05 to 0.08 mm., while the period was 

 such that the maximum of the deflection was reached in from 30 to 45 

 seconds, so that it usually required about 1.5 minutes to make a reading. 

 Such a slow period would militate against the use of a radiometer were 

 it not for the fact that the readings are always trustworthy, so that 

 nothing is gained by repeating them. 



The deflections also depend upon the dispersion, and the kind of radi- 

 ator used. For the small spectrometer, using acetylene, the maximum 

 deflections were about 40 cm. on a scale 1.4 meters distant, while with 

 the " heater " of a Nernst lamp they were from 10 to 15 cm. But the 

 latter gives out a stronger radiation from 4 to 10 fi, and is therefore the 

 more satisfactory. The radiation from this heater, as well as from 

 other clays, will be discussed in Appendix I. 



The advantages of the radiometer over the bolometer have been dis- 

 cussed by Nichols.^ The chief difficulties to be experienced in using a 

 radiometer is shifting of the zero, " drift," which was avoided by inclos- 

 ing the radiometer case (see fig. i) in a tin box, packed with wool. 

 This avoided all sudden changes of the " drift," which was then only in 



^Nichols, loc. cit., p. 302. 



