242 S. P. Langley — Bolometer. 



being usually about 10 millimeters long ; anywhere from 0*001 

 to 01 of a millimeter thick, arid, according to its special pur- 

 pose, being made from 1 millimeter to 0*1 millimeter wide. 



About 1886 the mounting of the instrument had been im- 

 proved by the writer, so that the strip appeared like the verti- 

 cal " wire " of a reticule in the focus of a positive eye-piece. 

 It was also movable in some cases by a micrometer screw, and 

 was, in fact, a micrometer thread controlled in the usual way, 

 but endowed with the special power of feeling the radiations 

 from any object on which it was directed. 



In the earliest spectrum work the bolometer developed 

 another important quality, its "precision." This quality is 

 quite independent of the accuracy with which it repeats meas- 

 ures of radiation or any constant source of heat, and concerns 

 the precision of setting, as a micrometer thread. It could 

 even twelve years ago, be pointed, not only like the thermo- 

 pile, within a fraction of a degree of the place of the source of 

 radiation, as for instance on a bright line in the spectrum, but 

 within a fraction of a minute of arc. 



The instrument of course depends for its general efficiency 

 on the galvanometer with which it is connected. That used 

 in 1886* had several improvements due to the suggestions of 

 Sir William Thomson and Professor Rowland, and was, per- 

 haps, at that time, the most effective instrument of its kind in 

 use for such a purpose, the mirror and needles having been 

 specially constructed at the Allegheny Observatory. The 

 mirrors were platinized by the kindness of Professor Wright, 

 and were at that time nearly a centimeter in diameter. The 

 needles were hollow magnets made by Mr. Very of the Alle- 

 gheny Observatory. For the damping mechanism of the 

 older galvanometer, I had substituted a dragon-fly (Libellula) 

 wing, in which nature offers a model of lightness and rigidity 

 quite inimitable by art. At that time, when making a single 

 vibration in 20 seconds, a deflection of one millimeter division 

 of the scale at one meter distance was given by a current of 

 0'000,000,0005 amperes, the instrument as described being 

 capable of recording a change of temperature in the bolometer 

 strips of less than 0*00001 of a degree centigrade. So much 

 less than this could be observed by special precaution, that it 

 might be said that this one one-hundred-thousandth of a degree 

 was not only indicable but measurable by the apparatus, which 

 was employed as described, in the determinations of the rela- 

 tions of n to X for the rock salt prism, and by which the infra- 

 red spectrum was at that time followed by actual measurement, 

 to a wave-length of rather over five one-thousandths of a milli- 

 meter. 



*This Journal, 3d series, vol. xxxii, page 90, 1886. 



