sorbed light. Light is absorbed by the block of glass, and the energy of the absorbed waves instead of appearing simply as heat, produces special molecular vibrations which give off waves of light, just as waves entering a harbor may set ships rocking and in consequence these become centers from which waves go out in all directions. Stokes announced the law that waves of fluorescent light cannot be shorter than the absorbed waves to which they are due.

It will be noticed that the block of glass fluoresces most strongly near the side where the incident beam enters, for as the beam penetrates into the block it loses by absorption the very rays which are effective in causing fluorescence.

The interposition of a piece of red glass in the path of the light cuts off all fluorescence, while a blue cobalt glass scarcely weakens it at all, showing that the effect is due to the shorter wave lengths which are transmitted by the blue glass but suppressed by the red.

Many substances show fluorescence, among others almost all mineral oils, especially the thick heavy oils, and crude petroleum, and even refined kerosene oil shows a delicate blue fluoresence in strong light. Some of the anilin substances are extremely fluorescent, notably fluorescein and eosin. Sulphate of quinine fluoresces a delicate blue as does also æsculin obtained from crushed horse-chestnut bark.

A white card covered with a thick paste of sulphate of quinine moistened with dilute sulphuric acid will fluoresce strongly in the invisible rays of the spectrum beyond the violet, the so-called ultra-violet region.

919. Phosphorescence. When fluorescence persists after the illumination ceases the substance is said to be phosphorescent.

By a special contrivance, called a phosphoroscope, Becquerel found that many substances, including paper, bone, and ivory, not usually known as phosphorescent, glow for a fraction of a second after the incident beam is cut off.

The sulphides of calcium, barium, and strontium are strongly phosphorescent and the color of the phosphorescent light is greatly influenced by the presence of slight impurities.

This kind of phosphorescence may be called physical to distinguish it from the glow of decaying vegetables, of fire-fly and

glow-worm, and of phosphorus itself, in which the light seems to be due to chemical changes.

920. Theory of Color Sensation.-The Young-Helmholtz theory of color sensation proposed by Thomas Young and modified by Helmholtz assumes that light falling on any point in the central region of the retina where it is sensitive to colors, excites in general three primary color sensations, red, green, and blue, the resulting color sensation depending on the relative intensities of these three primary sensations.

FIG. 565.-Curves from Abney, showing variation of color sensation with wave length, according to the Young-Helmholtz theory.

The sensation of red is found to be excited more or less by all wave lengths in the visible spectrum, but most strongly by the long waves, as shown in the left-hand curve of figure 565. So that if a person possessing only the red color sensibility and lacking those of green and blue were to look at a bright spectrum it would appear to him red from one end to the other, but brightest where the wave lengths are long as shown in the curve marked red. So, too, the curve marked green may be taken as exhibiting the relative intensity of the green sensation excited by different wave lengths of light, while the third curve shows how the sensation of blue varies with the wave length.

In the normal eye, possessing all three sensibilities, a given wave length of light excites all three sensations, the red predominating in case of long waves green when the waves are shorter, and blue when they are shorter still, the intensity of each sensation being proportional to the height of its curve at the point corresponding to the given wave length.

The sensation of white results when all three of the primary sensations are equally excited.

What the three primary color sensations are, can be determined only by the study of color-blind individuals. By such a study Koenig finds that the primary sensations are the red, green, and blue found in the spectrum at wave lengths, 671, 505, and 470, respectively. By combining these three colors in proper relative intensities any color of the spectrum may be produced.

Helmholtz assumed that there were three kinds of nerve termini in the retina corresponding to the three primary sensations of color, while Hering supposes certain substances in the retina whose transformations under the influence of light give rise to the various primary sensations. For a further discussion of theories of color vision the reader may consult "A First book in Pyschology" by Calkins, or the article "Vision" in Baldwin's "Dictionary of Philosophy and Psychology."

INTERFERENCE OF LIGHT.

921. Introduction.-Up to this point in our study the theory that light is a wave motion has been supported by the fact that the velocity of light is the same as that of electric waves and by the simple explanation which that theory affords of the phenomena of reflection and refraction. But we have not yet found any direct evidence of the existence in a beam of light of a regular periodic oscillatory motion such as is characteristic of all kinds of waves. We now come to some phenomena which point unmistakably to just such a periodicity.

922. Interference of Waves.-Perhaps the most distinctive evidence of wave motion is afforded by the phenomena of interference.

When two trains of waves come together having the same wave length and amplitude and traveling in nearly the same direction, there will be found points of rest or of very slight motion where the two systems of waves are in opposite phases and neutralize each other, and other points where the waves coming together in the same phase cause an amplitude of motion equal to the sum of the amplitudes of the component waves.

The interference of water waves and sound waves has already been discussed.

923. Young's Experiment.-The interference of light waves was first shown by Thomas Young in 1801 by the method illus

trated in the diagram. In the path of a beam of sunlight shining through a minute pinhole at S, is placed a screen of tinfoil having two very small holes a and b close together. If light is now allowed to pass through only one of the openings, a round bright spot surrounded by faint dark and bright rings is formed on a screen at C. But if light passes through both openings, there

a

FIG. 566.-Young's experiments showing interference.

is seen between the two bright spots and at right angles to their line of centers, a series of bright and dark bands, as shown in the lower part of figure 566.

The explanation of these bands will be understood by the aid of figure 567. Waves from S set up waves at a and b which start out simultaneously in the same phase, the two sets of waves spreading out in the medium beyond, one set from a and

FIG. 567.-Diagram of interference of waves. Young's experiment.

one from b, as shown in the diagram. The central point c is equidistant from a and b, so that waves leaving a and b at the same instant meet at c in the same phase, reenforcing each other and making c a bright spot. But d is a half wave length farther from b than from a, and consequently waves from a and b reach there in opposite phases and neutralize each other, making d

a dark spot. In this way those points on the screen which are equidistant from a and b, or which are one, two, or more whole wave lengths farther from one opening than from the other will be bright, while points which are farther from one opening than the other by or 1 or 2, etc., wave lengths will be dark.

924. Fresnel's Interference Experiment.-In order to show that the above explanation of the dark bands obtained by Young was correct and that they were really due to interference of waves, Fresnel devised a most ingenious modification of the experiment, by which he avoided any disturbance of the light that might be imagined to result from its passing through the small openings a and b.

Waves of light from a narrow slit shown in section at S (figure 568) fell on two mirrors M'M" inclined to each other at a

small angle so that light after reflection from M' diverged as if from S', while that reflected from M" came as if from S". In this way two trains of waves were produced which gave bright bands at c, f, and g where the waves of the two sets were in the same phase, and intermediate dark bands, just as in Young's experiment.

925. Newton's Rings and Colors of Thin Films.-When a lens having a convex surface of very slight curvature is placed in contact with a flat glass plate a thin film of air is enclosed between the two plates which increases in thickness from the