SMITHSONIAN INSTITUTION 



visible end of the red, in order to discover and map the Hnes of absorp- 

 tion there, produced by gases and vapors in the atmospheres of the 

 sun and of the earth. These infra-red rays are largely inaccessible to 

 photographic or visual observation, and were observed by the heat 

 they produce in that fine, blackened, electrically-connected strip of 

 metal called the bolometer which Langley invented. As published in 

 Volume I of the Annals of the Astrophysical Observatory, about 

 550 lines were found by the bolometer between the line called A 

 by Fraunhofer, which lies in the deepest red, and the great infra-red 

 band called O by Langley. 



The writer and Mr. Freeman set up more powerful apparatus and 

 examined this region anew. They found about 2,000 lines where 

 about 550 had been detected before. Their results are much ap- 

 preciated, and are being employed in various researches, notably by 

 Dr. Babcock of the Alount Wilson Observatory in his identifications 

 of the chemical elements represented in the sun by their infra-red 

 spectra. 



The writer continued his research, in cooperation with the Mount 

 Wilson Observatory, on the distribution of energy in the spectra 

 of the stars. Just as the color of the blacksmith's iron gives a notion 

 of its temperature, so, under carefully measured conditions, does 

 the study of the heat of different colors of their spectra give means 

 of estimating the temperatures of the stars. Furthermore, when both 

 the temperature and the distance of a star are measured, we may 

 estimate how large a star at that temperature and that distance must 

 necessarily be in order to send to the earth its observed total intensity 

 of radiation. 



Four years ago, the writer succeeded for the first time in observing 

 the spectra of 10 of the brightest stars in this way. He used the 

 radiometer, an instrument similar in principle to those sometimes 

 seen in rotation in sunlight in opticians' windows. In August and 

 September, 1928, he succeeded with a new form of the radiometer 

 in observing the energy of the spectrum of the planets Mars and 

 Jupiter, and of 18 stars, of which most were fainter than the second 

 magnitude, and one was of magnitude 3.8. 



The results were very accordant on successive nights, and will add 

 decidedly to our knowledge in this field. In the instrument used, the 

 vanes were made of house-flies' wings, about 1/75 inch wide and about 

 1/25 inch tall. The instrument was suspended in hydrogen at about 

 1/5000 ordinary atmospheric pressure by a quartz fiber too small to 

 be readil}' seen by the eye, even in selected lighting. Some idea of the 



