The apparent color of all objects is caused by the elective absorption of certain colored rays in the white light, while the remaining are reflected and determine the color of the object. Even the purest white and the most perfectly polished surfaces absorb some of the light. It is the same with the most transparent substances; they all absorb light more or less. In many of these an elective absorption also takes place; colored gems and glass or liquid solutions absorb certain colored rays and let others pass; those which pass determine the color of the substance. Sometimes, besides the absorption of several colors, a color is reflected complementary to that transmitted; in a thin layer of aniline red, red rays are transmitted, while green rays are reflected; a similar action takes place in a solution of litmus and several other substances. Some crystals possess the power of absorbing different colors when light passes-through them in different directions; this is called dichroism and polychroism. Thus the mineral iolite, a gem consisting of alumina, magnesia, and iron, shows different colors according as the light falls along the axis of crystallization or in a transverse direction.

Many artificial crystals exhibit the same remarkable property; for instance, the double chloride of platinum and potassium, which appears either deep red or bright green. The investigation of this peculiar kind of absorption of light has recently given rise to the invention of a new modification of the microscope by Haidinger, by which this property may be examined in the minutest crystals; this invention is called the dichroscope and dichroic microscope.

V. Absorption Spectrum. The elective absorption of transparent gases, liquids, and solids is determined by means of the spectroscope. This instrument proves in-deed that the cause of this absorption is simply the incapacity of the transparent substance to transmit luminous waves of a certain length, and thus that it is opaque for such waves. The result of such partial opacity is the formation of the so-called absorption bands, in case such a substance is placed between the light and the slit of the spectroscope. The Fraunhofer lines in the solar spectrum are in fact nothing but absorption bands produced by the passage of the light through the solar atmosphere; our own atmosphere also produces such bands, which spectroscopists call the atmospheric lines. The absorption spectrum differs in each substance which we may submit to examination. Thus iodine vapor and nitrous, acid vapor produce very characteristic absorption spectra when placed before the slit of the spectroscope (figs. 1 and 2), while different solutions of apparently the same color may be unmistakably distinguished from each other by the difference in the absorption spectra which they produce.

The most striking illustration is given by the black absorption bands produced by a perfectly clear and colorless solution of any salt of the rare metal didymium, so that in this way the merest traces of this metal in any solution may be detected, as lately found by Gladstone and Bunsen. Water, faintly colored yellow with a few drops of blood, may be distinguished from all other solutions of the same color, by showing in the spectroscope two characteristic absorption bands (fig. 3) in the green portion of the spectrum, not shown by any other substance; and it is even possible to recognize them in a single blood disk, by means of a microscope with spectroscopic eye piece. We add in fig. 4 the absorption bands of the solar atmosphere for comparison; they are used as landmarks to localize the absorption bands of other substances. They were first noticed by Wollaston, but afterward examined with such philosophical refinement by Fraunhofer, that they were named after him, and according to his proposition designated by A, B, C, D, etc. (See Specteoscope.)

RED.

VIOLET.

Fig. 1. - Absorption Spectrum of Iodine Vapor.

RED.

Fig. 2. - Absorption Spectrum of Nitrous Acid Gas.

RED.

VIOLET.

Fig. 3. - Absorption Spectrum of Blood.

RED.

VIOLET.

Fig. 4. - Solar Absorption Lines.