98 CORRELATION OF PHYSICAL FORCES. 



the many beautiful details into which the science of photo- 

 graphy has branched out, and the many valuable discoveries 

 and practical applications to which it has led. The short state- 

 ment I have given above is perhaps superfluous, as, though 

 the effects were new and surprising at the period when these 

 Lectures were delivered, photographic processes have no\v 

 become familiar, not only to the cultivator of science, but to 

 the artist and amateur : the important point for consideration 

 here is that light will chemically or molecularly affect matter. 

 Not only will the particular compounds above selected as in- 

 stances be changed by the action of light ; but a vast number of 

 substances, both elementary and compound, are notably affected 

 by this agent, even those apparently the most unalterable in 

 character, such as metals : so numerous, indeed, are the sub- 

 stances affected, that it has been supposed, not without reason, 

 that matter of every description is altered by exposure to light. 

 The permanent impression stamped on the molecules of 

 matter by light can be made to repeat itself by the same 

 agency, but always with decreasing force. Thus a photograph 

 placed opposite a camera containing a sensitive plate will be re- 

 produced, but if the size of the image be equal to the picture, 

 the second picture will be fainter in delineation than the first, 

 and so on. Thus again, a photograph taken on a dull day can- 

 not, by being placed in bright sunshine, be made to reproduce 

 a second photograph of the same size and more distinctly 

 marked than itself; I at least have never succeeded in such 

 reproduction, and I am not aware that others have : the image 

 loses in intensity as light itself does by each transmission. 

 The surface of the metal or paper may give a brighter image 

 from its being exposed to a more intense light, but the photo- 

 graphic details are limited to the intensity of the first impres- 

 sion, or rather to something short of this. A question of 

 theoretical interest arises from the consideration of these 

 reproduced photographs. We know that the luminosity of 

 the image at the focus of a telescope is limited by the area 

 of the object-glass. The image of any given object cannot 

 be intensified by throwing upon it extraneous light ; it is 

 indeed diminished in intensity, and when for certain purposes 

 astronomers illuminate the fields of their telescopes, they are 



