A Manual Of Photography

118 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. we have the violet ray: this would appear to be the blending of the red of the supplementary spectrum with the blue of the ordinary one, the lavender ray resulting from the intercombina- tion of the less luminous rays with the coloured surface upon which it is thrown. Then the extreme red or crimson ray will be seen to result from the blending of the extreme blue of the extraordinary with the red of the ordinary spectral image. This passage is still retained ; but I have every reason to believe that it will before long require some modification, the discovery of Mr. Stokes materially altering the conditions. Sir William Herschel, and Sir Henry Englefield, determined the heating powers of these rays to be very varied. A thermo- meter was placed in each, and the following results obtained. In the blue ray, in " green “ " yellow “ " full red “ 3' the thermom. rose from 55° to 56°, or 1° 3 3 2-r 21 “ edge of red" 21 Quite out of visible light in 21' 54 56 56 58 61 58 62 731 79 " 4 “ 6 “16 “ 151 “ 18 Sir John Herschel, by another form of experiment, has fully confirmed these results, and shown that the calorific or heat- producing radiations, being less refracted by the prism than the /^-exciting rays, exist a considerable distance further from the visible rays than has been hitherto suspected. Light and heat have not, therefore, the same degrees of refrangihility ; their influences are not coincident, their maxima in the solar spectrum* are wide asunder. Melloni has shown that, by the use of coloured media, these agencies can be, to a considerable extent, separated from each other. Glass stained with oxide of copper, and washed on one side with a colourless solution of alum, admits the light rays most freely, but obstructs 95 per cent, of the heat rays. On the contrary, a slice of obsidian or black mica obstructs nearly all the light radiations, but offers no im- pediment to the passage of heat. * The chemical influences of the prismatic rays vary as their heating powers, but in the contrary direction. If we place a piece of photographic paper in such a position that the spectrum falls upon it, it will be found to be very unequally impressed by the various rays. Some very extraordi- nary peculiarities have been observed by Sir lohn Herschel and myself; but it will be sufficient for our present purpose to state the general features of the impression under ordinary con- ditions. For some distance below the visible red ray, the paper will be found quite uncoloured ; on the part where the