ANTAGONIZING ACTION OF THE TWO ENDS OF THE SPECTRUM. 51 



frangible rays, which are included between the extreme red and the blue. There is 

 no difficulty, at any time, in securing the representation of the upper, but it is in this 

 lower part that the diversity of result is so great, and about which there is so much 

 difference of opinion among different authors. 



177. I regard these two classes of rays the more and the less refrangible as ex- 

 hibiting, upon the iodide of silver, antagonizing and contrary actions, the former exert- 

 ing a decomposing agency, the latter a protecting agency. The circumstances under 

 which the lower region makes its appearance are only found when the sky has that 

 degree of brightness that the sensitive surface is slightly stained by it ; the spectrum 

 rays then exert their protecting agency on the places on which they fall, and maintain 

 the iodide in an undecomposed state. That this is the true statement seems clear to 

 me, from the circumstance that the phenomenon is independent of time. It is immate- 

 rial whether we expose the sensitive surface for five minutes or for an hour, no change 

 whatever takes place in this lower region ; the iodide remains equally undecomposed. 

 Now this must arise from the circumstance that the decomposing effect of the skylight 

 is exactly balanced by the protecting agency of the lower rays so exactly balanced 

 that it is immaterial whether the exposure be for one minute or for an hour, the result- 

 ing action is the same. 



178. This view of the relation between the more and less refrangible rays, in the 

 action on iodide of silver, is strengthened by what is known to take place when phos- 

 phorescent surfaces are exposed to the sun. In that case, as all writers agree, the lesser 

 refrangible rays can not only exert a protecting agency, but even extinguish the phos- 

 phorescent glow occasioned by those which are more refrangible. 



179. Exposed to the prismatic spectrum, different compound substances give rise to 

 very different results. In some cases the change is limited to one, in others to other 

 regions of the spectrum. Thus the iodide of silver seems to be decomposed by the 

 more refrangible rays, the chloride exhibits a greater range of the same species of ac- 

 tion, the bromide is blackened by every ray from end to end of the spectrum. Of these 

 different results a very extensive series has been mad^feaowri. They have occupied 

 much of the attention of Sir J. HERSCHEL, who has published the results of his inves- 

 tigations in the Transactions of the Royal Society. 



180. Fig. 120 shows such impressions on different substances, a being the effect on 

 the iodide, and b on the bromide of silver. In these it is to be remarked, that not 

 only does the length of the figure vary very greatly, but also the position of the maxi- 

 mum point. 



