I 



236 PHOTOGRAPHIC PHENOMENA. [SECT. xxir. 



and much beyond it, in gradations of more or less intensity^ 

 it is found by careful investigation to be by no means con- 

 tinuous ; numerous inactive lines cross it, coinciding with 

 those in the luminous image as far as it extends : besides, a 

 very great number exist in the portions that are obscure, 

 and which overlap the visible part. There are three extra- 

 spectral lines beyond the red, and some strongly marked 

 groups on the obscure part beyond the violet; but the 

 whole number of those inactive lines, especially in the dark 

 spaces, is so great that it is impossible to count them. 



Notwithstanding this coincidence in the inactive lines of 

 the two spectra, photographic energy is independent of both 

 light and heat, since it exerts the most powerful influence 

 in those rays where they are least, and also in spaces where 

 neither sensibly exist ; but the transmission of the sun's light 

 through coloured media makes that independence quite evi- 

 dent. Heat and light pass abundantly through yellow glass, 

 or a solutionof chromate of potash ; but the greater part of the 

 chemical rays are excluded, and chlorine gas diluted with 

 common air, though highly pervious to the luminous and 

 calorific principles, has the same effect. Sir John Herschel 

 found that a slight degree of yellow London fog had a similar 

 effect with that of pale yellow media: he also remarked that 

 a weak solution of azolitmine in potash, which admits a great 

 quantity of green light, excludes chemical action ; and some 

 years ago the author, while making experiments on the trans- 

 mission of chemical rays, observed that green glass, coloured 

 by oxyde of copper about the 20th of an inch thick, excludes 

 the photographic rays ; and, as M. Melloni has shown that 

 substance to be impervious to the most refrangible calorific 

 rays, it has the property of excluding the whole of the 

 most refrangible part of the solar spectrum, visible and 

 invisible. Green mica, if not too thin, has also the same 

 effect, whereas amethyst, deep blue, and violet-coloured 

 glasses, though they transmit a very little light, allow the 



