204 Dr. Draper on some Analogies between 



of the light gradually disappears, the compound all the 

 time retaining its sensitiveness. 



(45.) Similar results are mentioned by Daguerre in the 

 case of the changes produced on surfaces of resinous bodies, 

 and I have noticed them in a variety of other cases. Now to 

 whatever cause these phaenomena are due, whether to any 

 thing analogous to radiation, conduction, &c, it is most active 

 during the first moment after the light has exerted its agency, 

 but it must also take effect even at the very time of exposure; 

 and it is for these reasons that it comes to pass, that when 

 light of a double intensity is thrown upon a metallic plate 

 the time required to produce a given effect is less than one 

 half. 



(46.) I could conceive the intensity of a ray so adjusted, 

 that in [falling upon a given sensitive preparation, the loss 

 from this cause, this casting off of the active agent, should ex- 

 actly balance the primitive effect, and hence no observable 

 change result. Hereafter we shall find, that one cause of the 

 non-sensitiveness of a number of bodies is to be traced di- 

 rectly to the circumstance, that they yield up these rays as 

 fast as they receive them. 



(47.) It needs no other observation than a critical exami- 

 nation of the sharp lines of a Daguerreotype proof with a 

 magnifying glass, to show that the influence of the chemical 

 rays is not propagated laterally on the yellow iodide of silver. 

 Of the manifestations which these rays may exhibit, after they 

 have lost their radiant form and become 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 in- 

 versely as their conducting power, we perceive at once why a 

 photographic effect, produced on yellow iodide of silver, re- 

 tains the utmost sharpness without any lateral spreading ; the 

 absorbing power is almost perfect, the conducting should 

 therefore be zero. 



(48.) 3rd. That, as when rays of heat fall on a mass of cold 

 ice, &c. &c. 



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 mercurialization, is in 

 direct proportion to the amount of incident light ; otherwise 

 it would hardly seem that the gradation of tones could be so 

 perfect. 



(49.) But in truth it is not so. When the rays begin to act 

 on it, the iodide commences changing, and is capable of be- 



