MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 



densed the mercury vapor, has the aspect of the high 

 lights of the daguerreotype, and forms, as it were, the 

 basis for the spectrum picture, there is in the region of 

 the more refrangible rays a bluish or olive-colored im- 

 pression, the counterpart of the result described in the 

 foregoing paragraph. But in the region of the less re- 

 frangible rays no mercurial deposit has occurred, the 

 place of those rays being depicted in metallic silver, 

 dark, and answering to the shadows of the daguerreo- 

 type. This protected portion, which stands out in bold 

 relief from the white background, reaches from a little 

 below G to beyond the extreme red, and encloses the 

 heat lines above named. They are in the form of white 

 streaks. Though I speak of them as single lines, they 

 are in reality groups, or perhaps bands. 



The general appearance of the photograph at once 

 suggests that the less refrangible rays can arrest the ac- 

 tion of the daylight and protect the silver iodide from 

 change. A close examination shows that there are three 

 points the extreme red, the centre of the yellow, and the 

 extreme violet which apparently can hold the daylight 

 in check. There are also two intervening ones in which 

 the actions conspire. The point of maximum protection 

 corresponds to the point of maximum action referred to 

 above in paragraph (1). 



(3.) If the metallic tablet, previously to its exposure 

 to the spectrum, be submitted for a few moments to a 

 weak light, so that, were it developed, it would at this 

 stage whiten all over, the action of the spectrum upon 

 it will be the same as in the last case (2). But this 

 change in the mode of the experiment leads to a very 

 important conclusion. The less refrangible rays can re- 

 verse or undo the change, in whatever it may consist, 

 that light has already impressed on the iodide of silver. 



Now, bearing in mind the fact that the photographic 



