418 BARNARD— ASTRONOMICAL PHOTOGRAPHY [April 20, 



cloud forms and structure of the Milky Way, for these in general 

 are very large. The field of view of a visual telescope, which is at 

 most but a mere speck of the sky, is entirely too small to take in the 

 whole of such an object. In the case of the Milky Way, the struc- 

 tural details are on such a grand scale that their true forms could 

 not even be guessed at with the ordinary telescope. The importance, 

 therefore, of the large field that the photographic plate gives us is 

 very evident. 



But there is one thing which we must take into account. The 

 time element enters strongly into the photographic part. One may 

 look into a telescope and he will see at once, if the conditions are 

 favorable, the faint star or faint nebula he has in the field of view — 

 it is but a moment that the eye takes to fix the image before it. 

 Perhaps some very faint and difficult object may require a little 

 longer, but it is only because a special moment of steadiness is 

 waited for. The photographic telescope with its highly sensitive 

 plate will not catch the object in that same time. It may require 

 an hour or more before it " sees " it. But with the eye there is 

 no cumulative effect ; on the contrary, indeed, it soon becomes tired, 

 so that in a sense, the longer you look the less you see, merely 

 from the fatigue of the eye. With the plate there is no fatigue. 

 The longer it looks the more it sees, so, though it may take it an 

 hour to see what the eye readily perceives in a moment, it does not 

 stop at that point but goes on seeing more yet, the longer it looks. 

 In this way it soon registers things that the eye cannot perceive at 

 all with equal optical means, and in many cases it reveals objects — 

 especially among the nebulae — that the eye may never see in the 

 actual sky. And, what is of immense advantage, it permanently 

 records what it sees, so that the exact appearance of a nebula may 

 be preserved for future reference perhaps hundreds of years hence, 

 while the view obtained by the eye is as evanescent as the fleeting 

 glimpse of the object itself. Even though the observer should make 

 a careful drawing it is too often worthless, and misleading, for 

 reference with other drawings, made later on; for the astronomer 

 is seldom or never an artist. 



If one examines drawings of the same celestial object by differ- 

 ent observers, he is often struck with the want of agreement in 



