88 Prof. Draper on the Existence and Mechanism 



In 1801 Ritter found that chloride of silver which had been 

 blackened in the violet rays, had its colour partially restored 

 when placed in the red. He states also that phosphorus, 

 which is oxidized with the production of fumes in the invisible 

 red, is instantly extinguished in the violet. 



The well-known experiments of Wollaston with guaiacum 

 served to show the opposite relations of the red and violet 

 rays. It is remarkable, that he subsequently abandoned this 

 interpretation of the phaenomenon, on discovering that green 

 guaiacum changed its colour by the application of a hot silver 

 spoon. 



In 1839 Sir J. Herschel encountered the same action in the 

 case of some of the preparations of silver. His first idea was 

 that of a positive and negative polarity of the spectrum ; but 

 this was subsequently modified for the reasons set forth in his 

 memoir*. 



From the time when I examined the Daguerreotype spec- 

 trum in Virginia in 1842, I have never doubted the actual ex- 

 istence of these negative or protecting rays ; and on this oc- 

 casion, when that existence is re-asserted by Lerebours, Fizeau 

 and Foucault, I will make known certain new facts, premising 

 that I do not think the views taken by M. Becquerel are cor- 

 rect. They are founded on what seems to me to be a misap- 

 prehension of the pheenomenon of the Daguerreotype. 



A Daguerreotype plate can exhibit three different varieties 

 of surface : — 1st, a black aspect on those regions where it has 

 been unaffected by light; 2nd, various shades of white; 3rd, 

 a coloured blackness, the tint of which may be of a deep 

 watch-spring, or sometimes of an olive shade. Persons familiar 

 with the process will understand completely what I mean. 

 The first of these conditions is represented in the deep sha- 

 dows 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 intensity 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 blackness, 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 pre- 

 cisely 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 second the 

 white state, and the third the solarized state. 

 ♦ Phil. Trans. 1840, § GO, kc. 



