paper in its utmost state of purity, it presents the appearance of a riband shaded with all the prismatic colors, having its breadth irregularly striped or subdivided by an indefinite number of dark and sometimes black lines. The greater number of these rayless lines are so extremely narrow that it is impossible to see them in ordinary circumstances. The best method is to receive the spectrum on the object-glass of a telescope, so as to magnify them sufficiently to render them visible. This experiment may also be made, but in an imperfect manner, by viewing a narrow slit between two nearly-closed window-shutters through a very excellent glass prism held close to the eye, with its refracting angle parallel to the line of light. When the spectrum is formed by the sun's rays, either direct or indirect, as from the sky, clouds, rainbow, moon, or planets, the black bands are always found to be in the same parts of the spectrum, and under all circumstances to maintain the same relative positions, breadths, and intensities. Similar dark lines are also seen in the light of the stars, in the electric light, and in the flame of combustible substances, though differently arranged, each star and each flame having a system of dark lines peculiar to itself, which remains the same under every circumstance. Dr. Wollaston and Fraunhofer of Munich discovered these lines deficient of rays independently of each other. Fraunhofer found that their number extends to nearly six hundred. From these he selected seven of the most remarkable, and determined their distances so accurately, that they now form standard and invariable points of reference for measuring the refractive powers of different media on the rays of light, which renders this department of optics. as exact an any of the physical sciences. The rays that

are wanting in the solar spectrum, which occasion the dark lines, are possibly absorbed by the atmosphere of the sun. If they were absorbed by the earth's atmosphere, the very same rays would be wanting in the spectra from the light of the fixed stars, which is not the case, for it has already been stated that the position of the dark lines is not the same in spectra from star-light and from the light of the sun. The solar rays reflected from the moon and planets would most likely be modified also by their atmospheres, but they are not, for the dark lines have precisely the same positions in the spectra, from the direct and reflected light of the sun.

A perfectly homogeneous color is very rarely to be found, but the tints of all substances are most brilliant when viewed in light of their own color. The red of a wafer is much more vivid in red than in white light; whereas, if placed in homogeneous yellow light, it can no longer appear red, because there is not a ray of red in the yellow light; and were it not that the wafer, like all other bodies, whether colored or not, reflects white light at its outer surface, it would appear absolutely black when placed in yellow light.

After looking steadily for a short time at a colored object, such as a red wafer, on turning the eyes to a white substance, a green image of the wafer will appear, which is called the accidental color of red. All tints have their accidental colors :-thus the accidental color of orange is blue; that of yellow is indigo; of green, reddish-white; of blue, orange-red; of violet, yellow; and of white, black; and vice versa. When the direct and accidental colors are of the same intensity, the accidental is then called the complementary color, because any two colors are said to

be complementary to one another which produce white when combined.

Recent experiments by Plateau of Brussels prove that direct and accidental colors differ essentially. From these it appears that two complementary colors from direct impression, which would produce white when combined, produce black, or extinguish one another by their union, when accidental; and also that the combination of all the tints of the solar spectrum produces white light if they be from a direct impression on the eye, whereas blackness results from a union of the same tints if they be accidental. M. Plateau attributes the phenomena of accidental colors to a reaction of the retina after being excited by direct vision. When the image of an object is impressed on the retina only for a few moments, the picture left is exactly of the same color with the object, but in an extremely short time the picture is succeeded by the accidental image. If the prevailing impression be a very strong white light, its accidental image is not black, but a variety of colors in succession. With a little attention it will generally be found that, whenever the eye is affected by one prevailing color, it sees at the same time the accidental color, in the same manner as in music the ear is sensible at once to the fundamental note and its harmonic sounds. The imagination has a powerful influence on our optical impressions, and has been known to revive the images of highly luminous objects months and even years afterwards.

SECTION XXI.

Newton and most of his immediate successors imagined light to be a material substance, emitted by all self-luminous bodies in extremely minute particles, moving in straight lines with prodigious velocity, which, by impinging upon the optic nerves, produce the sensation of light. Many of the observed phenomena have been successfully explained by this theory; it seems, however, totally inadequate to account for the following circumstances.

The same effect lengths be twice,

When two equal rays of red light, proceeding from two luminous points, fall upon a sheet of white paper in a dark room, they will produce a red spot on it, which will be twice as bright as either ray would produce singly, provided the difference in the lengths of the two beams, from the luminous points to the red spot on the paper, be exactly the 0.0000258th part of an inch. will take place if the difference in their three times, four times, &c., that quantity. But if the difference in the lengths of the two rays be equal to one half of the 0.0000258th part of an inch, or to its 12, 22, 32, &c. part, the one light will entirely extinguish the other, and will produce absolute darkness on the paper where he united fall. If the difference in the lengths of their paths be equal to the 11, 21, 31, &c. of the 0.0000258th part of an inch, the red spot arising from the combined beams will be of the same intensity which one alone would produce. If violet light be employed, the difference in the lengths of the two beams must be equal to the 0.0000157th part of an inch, in order to produce the same phenomena; and for the other colors the difference

must be intermediate between the 0.0000258th and the 0.0000157th part of an inch. Similar phenomena may be seen by viewing the flame of a candle through two very fine slits in a card extremely near to one another; or by admitting the sun's light into a dark room through a pin-hole about the fortieth of an inch in diameter, and receiving the image on a sheet of white paper. When a slender wire is held in the light, its shadow consists of a bright white bar or stripe in the middle, with a series of alternate black and brightly colored stripes on each side. The rays which bend round the wire in two streams are of equal lengths in the middle stripe; it is consequently doubly bright from their combined effect; but the rays which fall on the paper on each side of the bright stripe, being of such unequal lengths as to destroy one another, form black lines. On each side of these black lines the rays are again of such lengths as to combine to form bright stripes, and so on alternately, till the light is too faint to be visible. When any homogeneous light is used, suchas red, the alternations are only black and red; but on account of the heterogeneous nature of white light, the black lines alternate with vivid stripes or fringes of prismatic colors, arising from the superposition of systems of alternate black lines and lines of each homogeneous color. That the alternation of black lines and colored fringes actually does arise from the mixture of the two streams of light which flow round the wire, is proved by their vanishing the instant one of the streams is interrupted. may therefore be concluded, as often as these stripes of light and darkness occur, that they are owing to the rays combining at certain intervals to produce a joint effect, and at others to extinguish one another. Now it is con

It