REPORT OF THE SECRETARY 117 



continue radiometer measurements of the distribution of energy in 

 the spectra of the stars. This work was made possible by the availa- 

 bility of the 100-inch telescope of the Mount Wilson Observatory. 

 It had been i^roposed to substitute hydrogen for air in the radiometer, 

 on the theory that the radiometer reaction would be nearly the same, 

 but the damping and consequent sluggishness of action would be 

 much diminished in so light and free-moving a gas as hydrogen. 



Arriving in July, 1927, at Pasadena, the writer constructed the 

 radiometer vanes from bits of house-flies' wings. Incidentally it was 

 observed that it requires about 6,000000 house-flies' wnngs to weigh 

 one pound. With a fragment of microscope cover glass (ground and 

 polished to about one-third the usual thickness) the mirror of the 

 radiometer system was prepared. Two such systems of unequal, but 

 both of almost microscopic size, were hung upon quartz fibers so fine 

 as usually to be invisible, and were tested in air and in hydrogen at 

 various pressures. With them was used also a bolometric element 

 designed to give basis for an estimate of the comparative rise of tem- 

 perature of the radiometer vanes, when exposed to a constant source 

 of radiation, but contained in the different test gases. 



Hydrogen proved somewhat less efficient in regard to rise of tem- 

 perature and radiometer reaction than air, but abundantly justified 

 the expectation that its damping properties were much less objec- 

 tionable. On the Avhole, hydrogen appeared greatly superior as the 

 radiometer gas, and a carefully built system, with vanes 0.35 milli- 

 meters wide and 1 millimeter tall, was constructed. It had three 

 vanes in parallel on either side of the stem, separated 1 millimeter 

 between centers. This system was sealed into a glass ^ case in hydro- 

 gen under 0.2^) millimeter pressure of mercury. Provision had been 

 made to rotate the system by a magnetic device. 



After many trials, the device proved useless, because the mecha- 

 nism required to rotate the system so as to bring it to face in the 

 proper direction so stirred up the gas that wholly unexpected motions 

 resulted. After much labor the experiment was given up for the 

 year 1927. 



For use in 1928, at Doctor Adams's suggestion, there was prepared 

 an optically figured quartz cylindrical vessel. This fused quartz cyl- 

 inder, of beautiful clearness, was made to my order by the General 

 Electric Co., and Avas figured within and without at the Mount Wilson 

 Observatory shop. Being truly a circular cylinder with optically 

 figured concentric walls, it mattered not at all in what direction the 

 radiometer looked out. Thus by mounting the whole cylinder from a 



'I am grratly indcbti'd to tlie flirectdr ;incl s1;ilT of the Bureau of Standards, especially 

 Mr. Sperlins. and to tlic diroctor and stalT of tlie Mount Wilson Observatory, especiaUy 

 Mr. Porapt'o, for tlio construction of the special slass apparatus and the preparation for 

 Its use on Mount W^Uson. 



24034—29 9 



