The nature of sunlight.

Harper's new monthly magazine 1873

Dr. Draper, of New York, has lately published a summary of the views respecting the activity of the rays of the sun that have long been held by him, and which are now probably almost universally accepted by scientific men, although the elementary text-books on this subject have not yet been divested of the somewhat inaccurate expressions of thirty years ago, which latter also continue to be used by photographers and most practical men.

According to Dr. Draper, the calorific, luminous; and chemical effects produced by the solar rays are not so many distinct forces or emanations coexisting in a beam of light, and that can be dispersed by a prism, according to a fixed law, over the length of the spectrum, but are, on the contrary, only the various effects of one and the same force acting under different conditions and upon different substances. He maintains (1) that the chemical action is not limited to the more refrangible rays, but is equally distributed over the luminous and the calorific portions of the spectrum ; (2) that the ray effective in producing chemical or molecular changes in any special substance is determined by the absorptive power of that substance; (3) that there is also no special localization of the visual or the thermal effects.

In the case of the silver iodide so generally used by photographers, Draper shows that the more refrangible rays produce an effect contrary to that produced by the less refrangible. In the case of the bitumens and resins be shows that a properly prepared film of these is as sensitive to either the ultra red or the ultra violet rays as the silver iodide is to the latter rays only.

In the highly important case of the development of the carbonic acid gas found in the atmosphere by the action of sunlight on plants, he shows that this is accomplished by the action of the rays between the orange and the green bands of the spectrum, the maximum effect being in the yellow. The vegetable colors and the colors of flowers are shown to be dependent each upon the chemical action of a corresponding specific ray or rays. The union of chlorine and hydrogen goes on under the influence of every ray of the spectrum, but with greatest rapidity in the violet.

The effects of light on chlorophyl show that the vegetable colors are destroyed by rays complementary to those that have produced them.

The second of the above propositions is supported by the observations on the decomposition of the silver iodide, in relation to which Dr. Draper develops a fact of much interest to photographers, i. e., that the ordinary collodion him absorbs only about one-fourth of the whole actinic effect of the rays falling upon it; the rest passes through and is lost. Could the film be made to absorb the whole, its sensitiveness would be correspondingly increased.

The second proposition is especially supported by the direct experiments with chlorine and hydrogen. The solar rays having passed through a layer of chlorine are unable to cause the combination of a mixture of hydrogen and chlorine on which they are allowed to fall; without the intercepting layer of chlorine the solar rays cause the immediate combination of the two gases. Further experiments with absorbing media show that the more refrangible rays are the ones effective in causing the union of chlorine and hydrogen, and that, furthermore, the rays that are specially effective are those corresponding to the bands common to the spectra of the two gases. The process of union begins after the lapse of a certain time, during which the rays entering the mixture have been acting upon it to prepare it for the subsequent union. The actual union is a progressive phenomenon, the rapidity of which increases with the intensity and quantity of light.

The action of light on the chlorine compounds of silver is precisely similar to that on the compounds with hydrogen, and "there is to practical photographers an advantage, both as respects time and correctness, in light and shade gained by submitting a sensitive surface to a brief exposure in a dim light, so as to pass it through its preliminary stage."