modified by Coloured Glass Media, 8fc. 91 



quite an opposite effect upon the silvei* plate, whatever may 

 be the colour previously given to the surface by these two ra- 

 diations, endowing it with a property, the one of attracting, 

 the other of repelling the mercurial vapours. We must take 

 care not to confound these two resuUs; we can conceive two 

 difTerent actions giving the same colour to the iodide of silver, 

 and we can also conceive that these two actions may be en- 

 dowed with contrary properties as regards the fixation of mer- 

 curial vapour. 



The facts pointed out by M, Gaudin are the results of an 

 action which does not belong to the Daguerreotype, since they 

 are manifested without the aid of mercury ; for we must not 

 lose sight of the fact, that the production of the Daguerreotype 

 image is due onl}' to the affinity for mercury of the parts pre- 

 viously affected by the photogenic rays. It cloes not then 

 follow from the production of an image without mercury, by 

 crystallization or some peculiar arrangement of the molecules, 

 that the red, orange and yellow rays exert a continuing action 

 analogous to that which determines the fixation of mercurial 

 vapour. 



The experiments of Sir J. Herschel, of Dr. Draper, of M. 

 Lerebours, and of Messrs. Foucault and Fizeau, to prove the 

 protective and destructive action of the red rays, were made 

 with the prism. 



These philosophers have thus operated with the isolated 

 rays in all their natural purity, and after them it would have 

 been useless to seek to confirm or to contradict experiments 

 so ably conducted and so conclusive. 



Sir J. Herschel, in a memoir published in the Philosophical 

 Magazine for February 1842, approves only of experiments 

 made by means of the prism, as they are less subject to error 

 from the foreign rays, whicli the coloured glasses never en- 

 tirely exclude, This observation is perfectly just in theory, 

 but in practice, in the particular case of the photogenic power 

 of different rays and of their different actions, it will be found 

 that these phajnomena can be studied with greater facility by 

 using coloured glasses, and that the feeble quantity of foreign 

 rays which they admit, far from interfering with the deductions 

 of the experimenter, serve only to confirm and to render them 

 more conclusive. We shall presently see that these foreign 

 rays are completely neutralized in this class of experiments, 

 and it would have been unfortunate not to have added these 

 tests to those of the solar spectrum, since by the aid of coloured 

 glasses I have been enabled, not only to confirm certain pro- 

 perties of the pure spectrum, but also to discover some others 

 which had escaped my predecessors. 



