510 PROCEEDINGS OF THE AMERICAN ACADEMY 



by the displacement of the dark lines in the stellar spectrum. The 

 validity of the theory of this displacement was tested by applying it 

 to the eastern and western edges of the sun's equator, which are mov- 

 ing opposite ways with a known velocity. Professor Langley's plan 

 of testing the validity of his instrument, by taking the light first from 

 the two poles of the sun and then from the two edges of the equator, 

 was ingenious and convincing. What he did on this problem told in- 

 cidentally in his researches on radiant energy, assuring him of the 

 superiority in dispersive power of Rutherford's fine gratings over the 

 most powerful battery of prisms. Moreover his visit to Mount ^tna, 

 in 1878, enabled him afterwards to make a happy selection of a station 

 for his mountain observations. 



When Professor Langley began his work on solar energy, he found 

 that the thermo-multiplier did not give reliable results. In 1851 

 Svanberg had described in a volume of the Poggendorff Annalen a 

 new method of measuring heat by applying it so as to disturb the 

 electrical balance in Wheatstone's bridge, and he expressed the hope 

 that the new instrument would be found to rival, if not to exceed in 

 delicacy, the thermo-multipliers of Nobili and Melloni. This was the 

 only hint which Professor Langley had in the invention of his famous 

 instrument, the spectro-bolometer, the first description of which was 

 published in the Proceedings of this Academy for 1881. At that 

 time the instrument was from ten to thirty times more sensitive than 

 the thermo-multiplier, acted more promptly, and gave measurable re- 

 sults. Even the heat of the moon, on which the largest telescopes 

 had pronounced with no certain sound, surrendered to the touch of 

 the bolometer. 



Physicists were generally of the opinion that light and heat were 

 transmitted by separate undulations, or at least were independent 

 properties of the same undulation, as the maximum of light in the 

 prismatic spectrum is in or near the yellow, while the maximum of 

 heat is outside of the red. Dr. J. W. Draper predicted that the two 

 maxima would coincide in a normal spectrum, produced by a diffrac- 

 tion grating. But the heat from the grating was too small to be 

 measured by any instruments at his command, and he was able to 

 verify his prediction only to the extent of showing that the total heats 

 on the two halves of this spectrum were equal. At this juncture. 

 Professor Langley entered the field with his bolometer. He had at 

 his command two of Rutherford's gratings, ruled upon metal ; one 

 having 17206 lines to the inch (or 681 to the millimeter), the other 

 having half of that number. The rays from the grating fell upon a 



