PHYSICAL METABOLISM 243 



the shadow fell, shows that the loss of the diffused 

 light corresponding to the portion of the sky cut 

 off by the pole was quite sensible when that 

 portion lay very near the sun." 



" It is known that, with certain preparations, if a 

 plate be exposed for a very short time to diffuse day- 

 light, and be then exposed to a pure spectrum in a 

 dark room, on subsequently developing the image 

 it is found that while the more refrangible rays 

 have acted positively, that is, in the manner of 

 light in general, a certain portion of the less re- 

 frangible have acted in an opposite way, having 

 undone the action of the diffused daylight to which 

 the plate was exposed in the first instance. It 

 appears then that in photography, as in phosphor- 

 escence, there may in certain cases be an antagon- 

 istic action between the more and the less refran- 

 gible rays, so that it stands to reason that the 

 withdrawal of the latter might promote the effect 

 of the former." 



All this seems to show not merely that there is 

 a striking resemblance between photographs and 

 phosphorescence on the one hand, which we shall 

 have occasion to discourse upon, but also on the 

 other that the less refrangible rays have un- 

 doubtedly the power of destroying the phosphor- 

 escence as well as of preventing the phosphorescence 

 from taking place under the action of the more 

 refrangible rays. 



In the case of photography, however, Sir William 

 Abney, one of the highest authorities in scientific 



B 2 



