220 PROCEEDINGS OP THE AMERICAN ACADEMY 



colored silver salt owes its cohesion to the fact that the combined salt, 

 when exposed to light, has a molecular vibration, expressed in waves 

 of definite length. The addition of rays of light which may be in 

 discord or in accord with the vibration disturbs the cohesion, and 

 hence either the whole or part of the combining acid is set free. The 

 annexed diagram will serve to illustrate the relation between the 

 absorption and photographical action of some of the silver salts. 



By using a long focus collimating lens, practical experience shows 

 that not only do we obtain more illumination of the spectrum, but that 

 we can also bring into vision the ultra violet and red rays ; so that the 

 solar lines to L can be distinctly seen, as also the A lines. 



I 



