PHOTOGRAPHING THE STARS. 299 



pecially by the Messrs. Henry. Not only does the broad 

 division of the ring, usually known as Cassini's line, 

 appear very distinctly, but many of the more delicate 

 features are also perceptible. But the point which has 

 struck me very forcibly about this picture of Saturn is 

 the remarkable amount of shading which it gives to the 

 Saturnian globe. As is well known to every practical 

 astronomer, this globe usually possesses no very striking 

 varieties of shade or of colouration in the telescope, and 

 the extraordinary darkness about the poles of Saturn in 

 the photograph will arrest the curiosity of every one 

 who is familiar with the ordinary telescopic spectacle. 

 The cause of this phenomenon appears to lie not in the 

 actual colouration of the planet's globe, but in the atmo- 

 spheric shell within which it is contained. It would seem 

 that the Saturnian atmosphere, whatever be its character 

 in other respects, must at all events possess the power 

 of largely absorbing the photographic rays of light. 



At the present time the question of the application of 

 photography to the stellar regions is especially engrossing 

 attention, and for this purpose it would seem that the 

 new process is destined to effect a revolution in the art 

 of astronomical observation. We must therefore con- 

 sider the question of sidereal photography in some detail. 



When a telescope is directed towards a star, it brings 

 all the rays of that star to a focus ; and the more excel- 

 lent the construction of the optical part of the telescope, 

 the more accurately will the image of the star approximate 

 to that of a mathematical point. In the ordinary use 

 of a telescope for visual purposes, all the rays of light 

 collected by the aperture of the telescope are condensed 



