PHOTOGRAPHING THE STARS. 303 



are twofold. There is first the advantage of prolonged 

 exposure; there is secondly the possibility of utilising 

 invisible rays. 



The late Dr. Koberts, whose experience and marvellous 

 success in celestial photography entitle him to speak with 

 confidence on the matter, gives us striking evidence of 

 the detection of faint stars by the action of photography. 

 With an exposure of an hour he has shown on a plate of 

 about four square degrees a number of stars that he 

 estimates at more than sixteen thousand, of which the 

 brightest is less than the fifth magnitude. The circum- 

 stances appear to have been very favourable, for other 

 photographs have been obtained of the same region and 

 with exposures of equal duration. To all appearances the 

 nights on the three occasions were equally clear; but 

 clearness for visual purposes and clearness for photo- 

 graphic purposes involve different conditions ; and this is 

 remarkably illustrated by the three photographs referred 

 to. One of them, by Messrs. Henry, showed three thou- 

 sand stars ; the next, by Mr. Roberts, showed five thou- 

 sand stars ; while the third, by the same operator and on 

 the same region of the sky, disclosed more than three 

 times the number. 



It is of interest to attempt to estimate the total number 

 of stars visible to the photographic eye over the entire 

 surface of the heavens, assuming that the plate we have 

 just referred to may be taken as an average specimen of 

 the stellar richness of the entire firmament. The number 

 of square degrees in the heavens is about forty-one thou- 

 sand four hundred, and as the plate occupies four square 

 degrees, it will follow that upwards of ten thousand plate* 



