20 SMITHSONIAN INSTITUTION 



W. H. Hoover occupied the Smithsonian station on Mount Wilson, 

 May to October 1938, with two objectives: First, to grow plants in 

 nearly monochromatic rays, selected from the solar spectrum ; second, 

 to measure the distribution of radiation in the spectra of the brighter 

 stars by heat methods. 



Plant experiment. — Mr. Hoover used a very large Christiansen 

 filter in a solar beam reflected into the observatory from a coelostat. 

 In that way he selected a narrow band of radiation in the blue. Peas 

 grew in these nearly monochromatic rays for many weeks, until 

 reaching a height of nearly a foot. It was hoped that blossoms would 

 appear, but the experiment had to be closed before the plant was quite 

 ready to blossom. Check experiments were made in ordinary day- 

 light of about the same intensity. These check plants were a little 

 more robust, but the continued growth of the plants in monochromatic 

 rays for so long a period was surprising to many. 



Star spectra. — Several nights observing with the ioo-inch telescope 

 were attempted. Ten small Christiansen filters were used to select 

 narrow bands of spectrum at selected intervals between wave lengths 

 3400 angstroms in the ultraviolet and 10300 in the infrared. The 

 intensities were measured with a vacuum thermopile of L. B. Clark's 

 construction, connected to a vacuum, magnetically shielded, Thomson 

 reflecting galvanometer mainly of Hoover's construction. The mag- 

 netic shield was designed by the late Elihu Thomson of Lynn. The 

 galvanometer could be used in vacuum at 10 seconds single swing and 

 observed at 5 meters scale distance. At this sensitiveness, currents 

 of 1 x io -13 ampere, or one ten-trillionth of an ampere, would be 

 measurable. 



In practice, however, it proved that the thermoelectric couples 

 introduced too much drift for such extreme sensitiveness. The actual 

 observing of stellar spectra was done at not above 4 seconds single 

 swing of the galvanometer. But deflections as great as 20 millimeters 

 were observed in the spectrum of the star Vega. It is believed that 

 these experiments proved that when the 200-inch telescope is avail- 

 able, and with improved thermoelectric couples, and better insulation 

 from temperature changes of the surroundings, it will be readily 

 possible to use special diffraction gratings, and obtain continuous 

 energy spectrum curves of the brighter stars, with photographic regis- 

 tration of the galvanometer deflections. 



Hoover's curves are not yet all reduced. For the star Vega, how- 

 ever, his results are in very close agreement with those obtained by 

 Abbot in 1928 with the fly-vane radiometer. But Hoover observed 

 much farther toward the ultraviolet than Abbot, and he obtained more 

 than 10 times as large deflections. 



