TWENTY-FIVE YEAES' STUDY OF SOLAR RADIATION 



By C. G. Abbot 



Secretary, Smith sonian In-'^titution 



[With 3 plates] 



INFRA-RED SOLAR SPECTRUM MAP AND THE DISPERSION OF ROCK 



SALT 



Thirty years ago at Washington, under Dr. S. P. Langley's direc- 

 tion, F. E. Fowle and I completed a map of the sun's invisible infra- 

 red spectrum. Our map extended from Fraunhofer's "A," at wave 

 length 0.76 micron, to a point far down in the infra-red of wave 

 length 5.3 microns. We felt out and recorded the invisible spectrum 

 lines with the photographic registering spectrobolometer. Langley 

 used to say that in his use of the spectrobolometer on Mount Wliitney 

 in 1881 the indicator often raced across the scale 1 meter long in a 

 minute. We had so far tamed this wild creature by the year 1900 

 that the indicator seldom wandered a centimeter in an hour. Never- 

 theless, that delicate electrical thermometer, the bolometer, was then 

 so sensitive that a deflection of a millimeter on the photographic 

 recording scale corresponded to a temperature change of only 

 0.000004° Centigrade. 



The infra-red solar spectrum map which we made between 1895 

 and 1900 contained 740 lines. Their positions were recognized as 

 cooled bits of the spectrum by the fine metallic sensitive thread of the 

 bolometer. No doubt a considerable number of those 740 lines were 

 spurious, for every tremor of the eartli and every accidental temper- 

 ature change added its unwelcome deflections to the record. We 

 eliminated the false and preserved the true, as well as we were able, 

 by comparing many independent records. To fix the wave lengths 

 we made a special investigation of the dispersion of rock salt prisms. 

 Paschem repeated it later. His results agreed generally with ours 

 in the fifth decimal place of the refractive index of rock salt. 



In 1928 H. B. Freeman and I went over a part of this infra-red 

 spectrum again on Mount Wilson. We used three times as great 

 dispersion as in the old work at Washington. In the region from 

 0.76 to 1.8 microns we obtained about 1,300 lines where formerly we 

 found about 550. Dr. H. D. Babcock, of the Mount Wilson Observa- 

 tory, has done much photographic work covering a part of this upper 



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