34 THE WONDERFUL CENTURY. CHAP. v. 



intervals of time, we are able to study the motions of the 

 wings of birds, and thus learn something of the mechan- 

 ism of flight; while even the instantaneous lightning- 

 flash can be depicted, and we thus learn, for the first 

 time, the exact nature of its path. 



Perhaps the most marvellous of all its achievements 

 is in the field of astronomy. Every increase in the size 

 and power of the telescope has revealed to us ever more 

 and more stars in every part of the heavens; but, by the 

 aid of photography, stars are shown which no telescope 

 that has been, or that probably ever will be constructed, 

 can render visible to the human eye. For by exposing 

 the photographic plate in the focus of the object glass 

 for some hours, almost infinitely faint stars impress their 

 image, and the modern photographic star-maps show us 

 a surface densely packed with white points that seem 

 almost as countless as the sands of the seashore. Yet 

 every one of these points represents a star in its true 

 relative position to the visible stars nearest to it, and thus 

 gives at one operation an amount of accurate detail 

 which could hardly be equalled by the labor of an 

 astronomer for months or years even if he could ren- 

 der all these stars visible, which, as we have seen, he 

 cannot do. A photographic survey of the heavens is 

 now in progress on one uniform system, which, when 

 completed, will form a standard for future astronomers, 

 and thus give to our successors some definite knowledge 

 of the structure, and, perhaps, of the extent of the stellar 

 universe. 



Within the last few years the mechanical processes by 

 means of which photographs can now be reproduced 

 through the printing press have been rendered so per- 



