A Manual Of Photography

120 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY, itself in darkening, over the parts beyond the coloured image. However, there are two points where this change does not take place, and where the paper is preserved positively white ; these are tire points of maximum light and beat—the yellow and crimson rays. Here we have the evidence of the interference of these agencies with the chemical radiations. I have recently devised a more satisfactory experiment, which appears fully to prove that, although united in the sunbeam, light and chemical power do not belong to the same agency. As we can separate heat and light from each other by the use of coloured media, so can we isolate the chemical and luminous principles of the sun's rays. By a pure yellow glass we may cut off the agency pro- ducing chemical change so completely that the most sensitive photographic material may be exposed, covered by a glass stained yellow by oxide of silver, to a full flood of sunshine, without its undergoing any alteration in colour. If, however, we take a dark blue glass, such as is usually prepared with the oxide of cobalt, of so deep a colour that it obstructs a conside- rable quantity of light, and place under it the same, or any pho- tographic preparation, it will be found to darken as rapidly as if no glass had been interposed between it and the sun. Now, if we take a yellow glass stained throughout with oxide of silver, or a trough containing a solution of the bichromate of potash, and place it so that the prismatic rays must permeate it to reach the sensitive surface on which we desire to obtain the chemical spectrum, it will be found, if the glass is not of too deep a yellow, that very slight change has been made in the arrange- ment and relative sizes of the coloured bands of the spectrum. 15.