206 Dr. Draper on some Analogies between 



cident and absorbed, have been continually increasing ; but 

 as soon as the light thus latent has reached a certain quan- 

 tity, visible decomposition sets in, indicated by a blueness, and 

 the sensitive surface once more renders evident the increments 

 of incident light. 



(53.) Or, by presenting a plate covered with a screen to a sky 

 that is clear or uniformly obscured, and with a regular motion, 

 withdrawing the screen deliberately from one end to the other, 

 and then suddenly screening the whole ; it is plain that those 

 parts first uncovered will have received the greatest quantity 

 of light, and the others less and less. On mercurializing, it 

 will be seen that a stain will be evolved on the plate, as is 

 represented in fig. 5 ; from a to b the changes have been suc- 

 cessive ; from b to c no variation in the amount of whitening 

 is perceptible ; at d solarization is commencing, which be- 

 comes deeper and deeper to the end, e, of the stain. 



(54.) The plate from which the drawing of fig. 5 is taken, 

 gives from a to b ten parts, from b to c seventeen parts, from d 

 to e twelve parts; we perceive therefore how large an amount 

 of light is absorbed, and its effects rendered latent, between 

 the maximum of whiteness being gained, and solarization 

 setting in. 



(55.) 4th. That it depends on the chemical nature of the 

 jwnderable material what rays shall be absorbed. 



I had prepared a number of observations in proof of this, 

 very much of the same kind as those which have some time 

 ago been published in the Phil. Trans, by Sir J. Herschel. 

 These refer chiefly to the variable lengths of the stains, im- 

 pressed by the prismatic solar spectrum on different chemical 

 bodies, and the points of maximum action noticed in them. 

 For the present I content myself with referring to that ex- 

 cellent memoir for proofs substantiating this proposition. 



{56.) 5th. That whilst the specific rays thus absorbed de- 

 pend upon the chemical nature of the body, the absolute 

 amount is regulated by its optical qualities, such as depend 

 on the condition of its surfaces, and interior arrangement. 



I took a polished silver plate, and having exposed it to the 

 vapour of iodine, found that it passed through the following 

 changes of colour: — 1st, lemon yellow: 2nd, golden yellow; 

 3rd, reddish yellow : 4th, blue : 5tb, lavender : 6th, metallic : 

 7th, yellow: 8th, reddish: 9th, green, &c. &c. the differ- 

 ences of colour being produced by the differences of thick- 

 ness in the film of iodide, and not by any difference of che- 

 mical quality. 



(57.) It is a common remark, originally made by M. Da- 

 guerre, that of these different tints that marked 2 is the 



