stellar photography. 225 



Conclusions. 



Tlie work in stellar photography done at the Harvard College Observatory may 

 be summarized as follows. The first stellar photograph ever taken was obtained here 

 in 1850. In 1857 the investigation was resumed, and the value of stellar photography 

 as a means of determining the positions and brightness of the components of double 

 stars was established. In 1882, the present research was undertaken with a lens 

 having an aperture of only 21 inches. It was shown that photography could be 

 used as a means of forming charts of large portions of the sky, and of determining 

 the light and color of stars in all portions of the heavens. Photographs of the trails 

 of close polar stars no brighter than the eleventh magnitude were obtained without 

 clockwork. Stellar spectra were obtained of the brighter stars without clockwork, 

 in which all the principal lines were well shown. In 1885 the investigation was 

 resumed with a telescope having an aperture of 8 inches. With this, 117 stars within 

 one degree of the pole, one of them no brighter than the fourteenth magnitude, 

 left trails. The average deviation of the measures of the brightness of the.se stai's 

 on different photographs was less than a tenth of a magnitude, a greater accordance 

 than is given by any other photographic method. A similar result was obtained 

 from the Pleiades, of which group over fifty left trails. Similar trails are now being 

 obtained of the stars north of —30' in all right ascensions. This work began in the 

 autumn of 1885 at 23*, and has already been completed for more than half of the 

 sky. By photographing on the same plate polar stars near their upper and lower 

 culminations, material has been accumulated for determining the atmospheric absorp- 

 tion on each night of observation. A study has been made of the application of 

 photography to the transit instrument. Measurements of tlie trails show that the 

 position of a star may be determined from its trail with an average deviation of 

 COS, Avhich is about one half the corresponding deviation of eye observations. 



Charts may be constructed o" square, having the same scale and dimensions as 

 those of Peters and Chacornac. A single exposure of one hour is required, and it is 

 not necessary that the observer should remain with his eye at the telescope to correct 

 the errors of the clock. 



By placing a large prism in front of the object-glass, excellent stellar spectra have 

 been obtained. An exposure of five minutes gives the spectra of all stars brighter 

 than the sixth magnitude in a region 10° square. About half of the region north of 

 — 25', beginning at 0" 0", has been photographed in this way. "With an exposure of 

 an hour the spectra of stars no brighter than the ninth magnitude are shown. Over a 



