M.MMIK XVII. J ( IIKMK'AI- KAYS AM) KADIANT IIKAT. 235 



magnifying glass to show that the influence of the chem- 

 ical rays is not propagated laterally on the yellow iodide 

 of silver. Of the manifestation* which these rays may 

 exhibit, after they have lost their radiant form and be- 

 come absorbed, we know but little. If they conform to 

 the analogous laws for heat, and if the absorbing action 

 of bodies for this agent is inversely as their conducting 

 power, we perceive at once why a photographic effect 

 produced on yellow iodide of silver retains the utmost 

 sharpness without any lateral spreading; the absorbing 

 power is almost perfect, the conducting should therefore 

 be zero. 



3d. That, as lolien rays of heat fall on a mass of cold 

 ice, etc. 



Although in the sun the iodide of silver blackens at 

 once, this is only the result of a series of preliminary 

 operations. 



When we look at a daguerreotype, we are struck with 

 the remarkable gradation of tint, and we naturally infer 

 that the amount of whitening induced by mercurializa- 

 tion is in direct proportion to the amount of incident 

 light; otherwise it would hardly seem that the grada- 

 tion of tones could be so perfect. 



But in truth it is not so. When the rays begin to 

 act on it, the iodide commences changing, and is capable 

 of being whitened by mercury. Step by step this proc- 

 ess goes on, an increased whiteness resulting from the 

 prolonged action or increased brilliancy of the light, until 

 a certain point is gained, and now the iodide of silver 

 apparently undergoes no further visible change ; but an- 

 other point being gained, it begins to assume, when mer- 

 curialized, a pale blue tint, becoming deeper and deeper, 

 until it at last assumes the brilliant blue of a watch- 

 spring. This incipient blueness goes under the technical 

 name of solarization. 



