114 CORRELATION OF PHYSICAL FORCES. 



graph placed opposite a camera containing a sensitive plate 

 will be reproduced, but if the size of the image be equal to 

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

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

 cannot, by being placed in bright sunshine be made to repro- 

 duce a second photograph of the same size and more distinct- 

 ly 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 transmis- 

 sion. The surface of the metal or paper may give a brighter 

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

 photographic details are limited to the intensity of the first 

 impression, 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 in- 

 deed diminished in intensity, and when for certain purposes 

 astronomers illuminate the fields of their telescopes, they are 

 obliged to be contented with a loss of intensity in the telescopic 

 image. 



Now, let us suppose that the minutest details in the image 

 of an object seen in a given telescope, and with a given pow- 

 er, are noted ; that then a photographic plate is placed in the 

 focus of the same telescope so as to obtain a permanent im- 

 pression of the image which has been viewed by the eye-glass. 

 Could the observer, by throwing a beam of condensed light 

 upon the photograph, enable himself to bring out fresh details ? 

 or in other words, could he use with advantage a higher pow- 

 er applied to the illuminated photograph ? 



It is, perhaps, hardly safe to answer a priori this question ; 

 but the experiment of reproducing photographs would seem to 

 show that more than the initial light cannot be got, and that we 

 cannot expect to increase telescopic power by photography, 



