A Manual Of Photography

THE AGENT PRODUCING CHEMICAL CHANGE. 119 red ray falls, a tinting of red or pink will be evident. The orange and yellow rays leave no stain, and the green in general but a faint one. In the place occupied by the blue ray, the first decided darkening is evident, which increases througli the indigo and violet rays, and extends some distance beyond them. The shaded wood-engraving (Fig. 14) will serve Over this space the extra)....................... spectral blue rays occur.) Lavender .... Violet ...... Indigo ...... Blue......... Green........ Yellow....... Orange....... Red.......... Crimson...... 14. to assist the reader in comprehending the phenomena. The chemical radiations have a higher refrangibility than the ordi- nary luminous rays, and consequently they extend in full action to a considerable space beyond the lavender rays, where no light exists which can produce excitement on the optic nerve of the human eye, unless aided by some peculiar conditions of transpa- rent media, or of optical arrangements. "Whenever we throw a prismatic spectrum upon any photo- graphic surface, it is always accompanied by a sufficient quantity of diffused light to produce some chemical change, which shows