204 Mr. A. Claiulel on the Aclious of (he red and 



has therefore destroyed the action of daylight on the more 

 sensitive cuatino, and has continued tlie effect commenced on 

 the loss sensitive, to the same degree of intensity as the part 

 of the more sensitive coating which has not received the action 

 of red glass ; so that each half of the plate has an tlfect })er- 

 fectly ecjnal and identical, in every point, to that of the other 

 half, hnt in an inverted manner. A remarkable result is, that 

 the horizontal zone, which is precisely in the middle of the 

 plate, has the same intensity of photogenic effect in all its 

 length. This proves that there has been neither destruction 

 nor continuation on the space, where the coating by its thick- 

 ness had the mean state of sensitiveness. 



In putting aside the question of the influence of the various 

 periods of the year on the destruction or continuation of the 

 effect of daylight by the red and yellow rays, until we may 

 have been able to verily the exactness of the facts mentioned 

 by Dr. Draper, it is not possible to say in a general way that 

 the red and yellow glasses are endowed with the property of 

 continuing the effect of daylight on iodide of silver; for I 

 have just proved that there are cases in which the contrary 

 always takes place, according to the thickness of the coating of 

 iodine. 



Since the vapours of the atmosphere as well as the coloured 

 glasses render the action of light negative, in abs«trbing certain 

 rays and allowing only certain others to pass, it would not be 

 surprising that, from the simultaneous action of the vapours 

 of the atmosphere and of the coloured glasses, some contra- 

 dictory effects might result; that when liglit has to pass through 

 two different kinds of absorbing media, a certain effect could 

 be produced ; and that when the atmosphere is pure and free 

 from all kinds of vapour, the absorption of coloured glass only 

 might produce an effect of (juite an opposite nature. 



According to the position and density of the vapours of the 

 atmosphere, all the }K)ints of the luminous space aie not en- 

 dowed with the same })hotogenic properties. So that in some 

 circumstances a plate, first exposed to the blue light of the 

 zenith, loses the proj)erty of receiving the mercurial vapour, 

 if it be exposed a second time to the horizontal light of the 

 south, when there exist some va|5ours, although not sufficiently 

 dense to render the sun decidedly yellow. I have obtained 

 specimens in which this curious result is manifest. One of 

 them exhibits a negative image. This effect was produced 

 by exposing a plate, first to the blue light of the zenith, and 

 afterwards, covered with an engraving or a piece-of black lace, 

 to the light of the south. 



The possible simultaneous existence of two antagonistic 



