PHOTOGRAPHY AND ASTROPHYSICS 433 



transient impressions, is no match ultimately for the plate, which 

 can act by accumulation. But with similar instruments the plate 

 must be exposed for minutes or even hours to seize the impression 

 of a faint object which the eye can detect at a glance. There seems 

 to be no reason in the nature of things why the eye should not have 

 been surpassed in a few seconds; and in the future the sensitiveness 

 of plates may be increased so that this will actually be the case, even 

 as in the past there was a time when the sensitiveness was so small 

 that the longest exposure could not compete with the eye. But this 

 time is not yet come, and at the present moment the eye is still in 

 some departments superior to its rival, owing to this very fact, that 

 though it can only see by glances, it can use these glances to good 

 effect. In the study of the planets the more clumsy method of the 

 photographic plate (which, by requiring time for the formation of 

 the image, confuses good moments with bad) renders it almost use- 

 less as compared with the eye; and again, we have not as yet used 

 photography for daylight observations of stars. 



But there is another direction in which the photographic plate 

 is immensely superior to the eye in power; it can record so much 

 more at once. 1 In the able hands of Prof. Barnard, Dr. Max Wolf, 

 and others, this property of the plate has been used to record the 

 presence in the sky of vast regions of nebulosity such as, we may 

 safely say, the eye would never have satisfactorily portrayed, not 

 altogether because of their faintness (for in one of his papers Professor 

 Barnard tells us that he was actually led to photograph such a region 

 because he had become vaguely conscious of it by eye-observation), 

 but because of their diffusion. It is noteworthy that these beautiful 

 photographs were taken with comparatively humble instruments, 

 and we may be as yet only on the threshold of revelations still to 

 be made in this direction. 



Secondly, the photographic method represents a great advance 

 in facility of manipulation. A familiar example may be taken from 

 the domain of planetary discovery. In old time, to recognize a new 

 object among numerous fixed stars, it was necessary either laboriously 

 to map out the whole region, or to learn it by heart, so that it was 

 practically mapped in the brain. Now all this labor is avoided; two 



1 This property has been beautifully illustrated by a lecture experiment of 

 Prof. Barnard. He throws on the screen a picture of a large nebula which the 

 photographic plate has no difficulty in portraying all at once; but the picture is 

 in the first instance covered up by a screen, except for a small aperture only, and 

 this aperture, he tells his audience, represents all that can be seen by the eye at 

 one time, using the giant telescope 'of the Yerkes Observatory. By moving the 

 screen about, different portions of the picture may be viewed successively, as also 

 by moving the telescope about in looking at the sky itself. But what a revelation 

 follows when the screen is removed and the full glory of the nebula is exhibited 

 at a single glance 1 We can well understand that the true character of these objects 

 was hopelessly misinterpreted by the eye, using the imperfect method of piecemeal 

 observation which alone was formerly possible. 



