PHOTOGRAPHING THE STARS. 301 



is permitted, and thus we see one of the reasons why 

 photography forms such an admirable method for depicting 

 the stars. We can give exposures of many minutes, or of 

 one, two, three, or even four hours ; and all the time the 

 effect is being gradually accumulated. Hence it is that 

 a star which is altogether too feeble to produce an im- 

 pression upon the most acute eye fortified by a telescope 

 of the utmost power may yet be competent, when a 

 sufficient exposure has been allowed, to leave its record on 

 the plate. Thus it is that photographs of the heavens 

 disclose to us the existence of myriads of stars which 

 could never have been detected except for this cumulative 

 method of observation which photography is competent 

 to give. 



There is another peculiarity about the photographic 

 methods of observation which gives them an importance 

 from quite a distinct point of view. The radiation from 

 a star consists of a number of rays of very varied hues 

 all blended together. If they were separated out, we 

 should find that they were divisible into two great groups 

 namely, the visible and the invisible. As to the former, 

 they characterize the well-known hues of the rainbow : 

 the red, the orange, and the yellow, the green, blue, indigo, 

 and violet. It is to these rays in varying degrees of 

 combination that we are indebted for visibility in the star, 

 either to our unaided eye, or even to the eye aided by a 

 telescope. But it is conceivable that a star might dispense 

 a rich stream of rays, and yet be totally invisible from 

 the fact that none of those rays belonged to the special 

 group which can alone excite vision. These invisible rays 

 may be of different types. Some of them might be rays 



