THE FUNCTION OF LARGE TELESCOPES. 133 



their changes in brightness. In this way he has confirmed the varia- 

 bility of many of the stars on Mr. Bailey's photographs. There are few 

 more remarkable objects in the heavens than these magnificent star 

 clusters, so many members of which are subject to fluctuation in their 

 light. Professor Bailey's discovery is the more noteworthy considering 

 the fact that such an object as the great cluster in Hercules contains 

 not more than two or three variable stars, while the Harvard plates 

 show that the cluster Messier 3 contains 132 variables. This is only 

 one instance out of many of the striking efficiency of the photographic 

 work which is being carried on under Professor Pickering's able 

 direction. 



It may be well to introduce here a few words regarding the magnify- 

 ing powers employed in actual observations. The optimistic writer 

 Avho is planning to photograph houses on Mars believes that his recent 

 invention will render possible the use of powers as high as a million 

 diameters, and even greater, so that if men exist upon the planets they 

 can easily be seen. Astronomers know nothing of such powers ill 

 practice. For double-star observations, with the largest telescope and 

 under the most perfect conditions, powers as high as 3,700 diameters 

 have occasionally been used. But in regular work it is not a common 

 thing to exceed 2,700 diameters. Under very exceptional circum- 

 stances the moon might perhaps be well seen when magnified 2,000 

 diameters, but this would be an extreme case, and in general a much 

 better view could be had with powers ranging from 500 to 1,000. 

 Jupiter can rarely be well seen with a power greater than four or five 

 hundred, though Saturn will stand considerably higher magnification. 

 Mars is best seen with a power of five or six hundred. With small 

 telescopes lower powers are generally used. The difficulty is not in 

 finding optical means to increase the magnification, as some of these 

 newspaper writers seem to imagine. It is rather a question of being 

 able to see anything but a confused luminous object after the high eye- 

 pieces had been applied. The more or less disturbed condition of the 

 earth's atmosphere is mainly responsible for this, but it is doubtful 

 whether, with even perfect conditions, such an object as Jupiter could 

 be advantageously submitted to great magnification. 



During the present century there has grown up side by side with 

 astronomy, to which it in fact owes its existence, the new science of 

 astrophysics. In a broad sense this science may properly be classed 

 as a department of astronomy, but at the present time its interests are 

 so manifold, its methods so distinct, and its relationship to pure 

 physics so pronounced, that it may fairly claim to be considered by 

 itself as a coordinate branch of science. While astronomy deals more 

 especially with the positions and motions of the heavenly bodies, it is 

 the province of astrophysics to inquire into their nature and to search 

 out the causes for the peculiar celestial phenomena which the special 

 instruments at the disposal of the astrophysicist bring to light. It 



