MEMOIK V.] INTERFERENCE OF KADIATIONS. 89 



gions where it has been unaffected by light ; 2d, various 

 shades of white; 3d, a colored blackness, the tint of 

 which may be of a deep watch-spring lustre, or some- 

 times of an olive shade. Persons familiar with the proc- 

 ess will understand completely what I mean. The first 

 of these conditions is represented in the deep shadows 

 of such a photograph, the places where the light never 

 acted ; the second is exhibited in the various intensities 

 of whiteness, which constitute the figures of the picture, 

 the whiteness varying in intensity according to the in- 

 tensity of the light; the third is the solarized or overdone 

 condition, which arises from too long an exposure to the 

 rays. Like the first, this may be spoken of as a black- 

 ness, but in reality it is a dark green or blue or tawny 

 tint. It is this solarized condition of surface which M. 

 Becquerel confounds with the first, the blackness arising 

 from the unchanged state; and it is precisely on this 

 point that the whole argument turns. For the sake of 

 having distinctive words to mark out these three con- 

 ditions, I will call the first the unaffected state, the sec- 

 ond the white state, and the third the solarized state. 



The observations I made in Virginia were as follows : 

 That if a solar spectrum be received on a daguerreotype 

 plate on which a weak daylight was simultaneously 

 acting, the red, orange, yellow, green, and part of the 

 blue rays arrested the action of the daylight on that 

 portion of the plate on which they fell, and maintained 

 it in the unaffected state; while the residue of the blue, 

 the indigo and violet, carried their part of the plate to 

 a completely solarized condition. This therefore seemed 

 to justify the assertion that the less refrangible rays 

 protect Daguerre's preparation from the action of a dif- 

 fused daylight. 



It was also found that if the plate were exposed to 

 the daylight for a few seconds, so that had it been then 



