If a solution of silver nitrate is washed over a large sheet of paper, which is then placed upon the wail or screen so as to receive the spectrum upon its surface, and is also made to cover the space considerably above it, a transformation occurs where the radiations fall, producing a blackening which defines the outline of the chemical spectrum. It is now found that the chemical rays are more refrangible than the luminous, and that, while the darkening takes place in the colored spectrum, it it strongest in the violet of all the colors, and extends also through the dark space up to B, as shown in the figure.
It is now exactly 200 years since Newton published his "Optics," in which was described the capital experiment of resolving white light into its constituent colors by the prism. It was the first great step toward showing that what was regarded us perfectly simple turns out to be inexhaustibly complex; and every succeeding step of research, while clearing up some points, has led to others which are still unresolved. One thing, however, seems to be quite clear: the mode of action throughout the spectrum is fundamentally the same. There are three spectra, one of which, the thermal, takes action upon all kinds of matter; another of which, the luminous, acts only upon a certain special form of nerve-matter; while a third, the chemical, produces changes in certain compounds. Although the luminous force acts only upon the nerve of the eye to stir up a sensation, yet we know how infinitely complex and varied is the world of color that results. There is evidence that the dark thermal and chemical radiations are of equal variability and complexity, yet there can be no doubt that all these multitudinous effects are due to a single mode of action. The difference between the thermal and the chemical rays is simply the difference between the red and the green; that is, a difference of wavelength and degree of vibration.
The unequal distribution of the forces of the spectrum is well illustrated by Fig. 2. The middle curve shows the varying intensity of the luminous force. The maximum is at B, in the yellow space; and from this point the intensity of the light rapidly declines each way, its extent being shown by the space shaded with oblique lines. The curve A, with the vertical lines, represents the position and varying force of the heat; and the curve C, horizontally shaded, exhibits the distribution and unequal energy of the chemical force. The three maxima are widely separated as if there were some antagonism among them, and it is noticeable that where the light is strongest the chemical force quite disappears. Different prisms give somewhat different effects but do not change their order.
It thus appears that, so far from light being the agent which produces sun-pictures, the intensest light is powerless upon the chemically prepared plate. It looks as if the illumination neutralized or extinguished the chemical energy. Nevertheless, light and the chemical force are so intimately associated in reflection and refraction that
