PHOTOGRAPHING THE STARS. 295 



property from visual transparency. By steadiness is 

 meant such a regularity in the variations of density that 

 each ray of light is persistently refracted along the same 

 course throughout the duration of the exposure. By 

 transparency the celestial photographer will mean a state 

 of the air which will permit the particular rays of light 

 which he wants to pass through. It will often happen 

 that two nights which to the unaided eye, or even in the 

 ordinary telescope, seem equally clear, may be of widely 

 different clearness in so far as the photographic light is 

 concerned. 



To illustrate the opacity of the atmosphere to photo- 

 graphic rays I may mention a fact told me by the Rev. 

 H. Swanzy, who accompanied the Rev. W. Green on his 

 recent exploration of the Selkirk range in British Colum- 

 bia. The plates they used required an exposure of three 

 seconds or more in the valleys, while similar plates 

 exposed at a height of ten thousand feet were found to 

 be destroyed if the exposure was more than a small 

 fraction of a second. 



In the application of photography to celestial por- 

 traiture, we naturally first allude to the photographs of 

 the sun, by far the most exquisite of which are those taken 

 by Janssen at Meudon. His photographs, obtained by 

 an extremely short exposure of a fraction of a second, 

 display in a marvellous manner the actual texture of the 

 sun under the conditions of its surface at the moment. 

 They prove that the luminous parts are brilliant granules 

 or cloudlets, floating, so to speak, in a less bright medium 

 which is visible in the interstices between the cloudlets. 

 Occasionally the openings between the small luminous 



