OPTICAL QUALITIES CONTROL CHEMICAL ACTION. 153 



plate, as is represented in Jig. 97 ; from a to b the changes have been successive ; from 

 b to c no variation in the amount of whitening is perceptible ; at d solarization is com- 

 mencing, which becomes deeper and deeper to the end, e, of the stain. 



620. The plate from which the drawing of jig. 97 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 maxi- 

 mum of whiteness being gained and solarization setting in. 



621. 4th. That it depends on the CHEMICAL nature of the ponderable 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 Philosophical Transactions 

 bv Sir J. HERSCHEL. These refer chiefly to the variable lengths of the stains, impress- 

 ed by the prismatic solar spectrum on different chemical bodies, and the points of max- 

 imum action noticed in them. For the present, I content myself with referring to that 

 excellent memoir for proofs substantiating this proposition. 



622. 5th. That while the specific rays thus absorbed depend 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 arrangetnent. 



623. 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 ; 2d, 

 golden yellow ; 3d, reddish yellow ; 4th, blue ; 5th, lavender ; 6th, metallic ; 7th, yel- 

 low ; 8th, reddish ; 9th, green, &c., &c., the differences of colour being produced by 

 the differences of thickness in the film of iodide, and not by any difference of chemical 

 quality. 



624. It is a common remark, originally made by M. DAGUERRE, that of these dif- 

 ferent tints that marked 2 is the most sensitive, and photogenic draughtsmen generally 

 suppose that the others are less efficient from the circumstance of the film of iodide 

 being too thick. Some suppose, indeed, that the first yellow alone is sensitive to light 

 We shall see, in a few moments, that this is very far from being the case. 



625. Having brought nine different plates to the different colours just indicated, I re- 

 ceived on each the image of a uniform gas flame in the camera, treating all as nearly 

 alike as the case permitted. I readily found that in No. 1 there was a well-marked 

 action ; No. 2, still stronger, but that the rays had less and less influence down to No. 

 6, in which they appeared to be almost without action ; but in No. 7 they had recovered 

 their original power, being as energetic as in No. 2, and from that declining again ; this 

 is shown mfig. 98. 



626. Hence we see that the sensitiveness of the iodide of silver is by no means con- 

 stant : that it observes periodical changes, which depend on the optical qualities of the 

 film, and not on its chemical composition ; and that by bringing the iodide into those 

 circumstances that it reflects the blue rays, we greatly reduce its sensitiveness, and still 

 more so when we adjust its thickness so as to give it a gray metallic aspect. But the 

 moment we go beyond this, and restore by an increased thickness its original colour, 



U 



