424 Dr. J. W. Draper on the Distribution of 



reaches from a little below G to bevond the extreme red, and en- 

 closes 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 action of the day- 

 light, and protect the silver iodide from change. A close exa- 

 mination shows that there are three points, the extreme red, 

 the centre of the yellow, and the extreme violet, which appa- 

 rently can hold the daylight in check. There are also two in- 

 tervening 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 

 reverse or undo the change, in whatever it may consist, that 

 light has already impressed on the iodide of silver. 



Now, bearing in mmd these facts, that the photographic action 

 of diffused light on this iodide is mainly due to the more re- 

 frangible rays it contains, we are brought by these experiments 

 to the following conclusions :— 



1st. Every ray in the spectrum acts on the silver iodide. 



2nd. The more refrangible rays apparently promote the action 

 of the daylight on that substance ; the less refrangible apparently 

 arrest it. 



3rd. For the display of this arresting or antagonizing effect, 

 it is not necessary that the less and more refrangible rays should 

 be acting simultaneously. An interval may elapse, and they may 

 act successively. Hence the effect is not due to the contempo- 

 raneous interference of waves of different periods of vibration 

 with one another; the material particles of the changing sub- 

 stance of the silver iodide are involved. 



I abstain for the moment from giving further details of these 

 spectrum impressions. That has been very completely done by 

 Herschel, in the case of one I sent him many years ago. His 

 examination of it, illustrated by a lithograph, may be found in 

 the Philosophical Magazine (Feb. 1843). I shall have to 

 return to the subject of the behaviour of silver iodide in. pre- 

 sence of radiations on a subsequent page of this memoir. 



The main point at present established is this : — that the silver 

 iodide, under proper treatment, is affected by every ray that a 

 flint-glass prism can transmit; and therefore it is altogether 



