298 LIGHT SCIENCE FOR LEISURE HOURS. 



and so forth, which, like the telescope and microscope, 

 are akin to the organ of vision, but give to it increased 

 power in particular ways. 



I have lately been led to notice how certain photo- 

 graphic processes and methods extend the powers of 

 human vision, and enable us to see what, owing to 

 certain peculiarities in the circumstances under which 

 eyesight is employed, we are debarred from seeing in 

 the ordinary way. 



It has long been noticed that photographic vision, 

 so to describe this method of studying natural objects, 

 has one great advantage over ordinary vision in that it 

 is not liable to ordinary misleading influences. In 

 science, seeing is not always, or even generally, 

 believing for the simple reason that the student of 

 science cannot always be certain what he really sees. 



Thus an observer may be misled by imagination, 

 especially if some favourite theory has possession of his 

 mind. If he knows, or thinks he knows, what he ought 

 to see, or might fairly expect to see, he is very apt to 

 imagine that he actually does see it. In this way, for 

 instance, many students of astronomy have fancied 

 they have seen a small companion by a star in a position 

 where they had been told such a companion existed, 

 when, in reality, there had been some error in the 

 description, or in their reading of it, and either no 

 such companion existed, or else it was in some entirely 

 different position, and perhaps quite beyond the range 

 of the telescope employed by the observer. 



Again, the eye is repeatedly deceived by effects of 



