STELLAR PHOTOGRAPHT. 181 



These experiments were soon after repeated bj Mr. De la Rue and by Mr. 

 Rutherfurd. A much more extended investigation was undertaken in 1864 by 

 Mr. Rutherfurd, and continued by him during many years. A brief notice of this 

 work was published in the American Journal, XXXIX., 1865, 304. One of his 

 photographs of the Pleiades was measured and discussed by Dr. B. A. Gould, in 

 1866, in the Astrouomische Nachrichten, LX^'III. 184. A list of the clusters pho- 

 tographed by Mr. Rutherfurd is given by Professor Holden, in the Smithsonian Mis- 

 cellaneous Collections, 311, page 89. The faintest stars shown in these photographs 

 are probably not far from the ninth magnitude. By the continued improvement 

 in photographic processes, each experimenter after a few years has a great advantage 

 over his predecessors. The invention of dry plates simplified the work of taking a 

 photograph, permitted an indefinite prolongation of the time of exposure, and ulti- 

 raatel}- greatly increased the sensitiveness of the film. Aided b}^ these advantages, 

 Dr. Henry Draper attacked the problem with his usual skill and perseverance. On 

 March 11, 1881, he obtained a photograph of the Nebula of Orion, in which a star 

 is .shown whose photometric magnitude is about 14.7. This star is barely visible 

 with a telescope of the same aperture as that with which the photograph was taken. 

 The photographic plate, accordingly, had now become as efficient an instrument of 

 research as the eye itself, by means of its power of accumulating the energy radiated 

 upon it. For a further discussion of the faintest stars thus photographed, see Wash- 

 ington Observations, 1878, page 226, and Proceedings of the American Academy, 

 XX. 407. But for Dr. Draper's untimely death, he would doubtless have been the 

 first to accomplish the striking experiment of photographing a star too faint to be 

 seen in the largest telescopes. This result was apparently soon after attained by Mr. 

 A. A. Common, in his beautiful photograph of the Nebula of Orion. It is not safe to 

 draw conclusions from a single star, on account of the effect of color. A red and 

 a blue star may produce photographic images of equal intensity, although to the 

 eye their brightness may diflFer b}' several magnitudes. A comparison of the photo- 

 graph of Mr. Common with the catalogue of Bond (Annals Harvard College Obser- 

 vatory, V. 270) has been made by the writer (Proc. Amer. Acad., XX. 407). From 

 this it appears that the number of stars contained in the photograph and not given 

 in the catalogue is about equal to the number of stars in the catalogue which are 

 wanting in the photograph. We may therefore, conclude that the limiting magni- 

 tude for the photograph does not differ greatly from that of the faintest stars visible 

 to the eye. The largest telescopes add but few stars to those given by Bond, 

 although this nebula has been thoroughly scrutinized with the reflectors of the 



