PHOTOGRAPHING THE STARS. 311 



through the guiding telescope. A star is chosen, and 

 this star is brought on the intersection of a pair of spider 

 webs in the guiding telescope. During the entire expo- 

 sure this star must remain in the same position, and this 

 the attending astronomer will secure by gently correcting 

 the speed of the driving clock. When this fiducial star 

 has been kept carefully on one point throughout the 

 exposure, then, assuming that other obvious conditions 

 are fulfilled, each star will have been constantly brought 

 to a focus on the same point of the photographic plate. 

 The condition is a somewhat trying one, when we remem- 

 ber that the image of the star is an extremely small point, 

 and that the duration of the exposure is in some cases as 

 long as four hours, or even more. 



A combined effort has been made to secure a repre- 

 sentation of the entire surface of the heavens by photo- 

 graphy. A congress met in Paris, under the presidency 

 of Admiral Mouchez, consisting of astronomers from all 

 parts of the world, and the conditions under which this 

 stupendous survey of the universe was to be undertaken 

 were then decided on. The operations are divided among 

 a number of observatories situated over the world, and 

 each of them undertakes to photograph on plates of a 

 uniform size a certain region of the heavens. The work 

 has been entered upon with the heartiest enthusiasm, and 

 ere many years have elapsed we may anticipate being in 

 possession of what will practically be a photograph of the 

 entire heavens. This great piece of work will provide us 

 with the means of making a reasonably complete inven- 

 tory of the entire contents of that small portion of the 

 universe which lies within the reach of our instruments. 



