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. The rayless lines in the red portion of the spectrum become most visible as the sun approaches the horizon, while those in the blue extremity are most obvious in the middle of the day. 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 M. Fraunhofer of Munich discovered these lines deficient of rays independently of each other. M. Fraunhofer found that their number extends to nearly six hundred. There are bright lines in the solar spectrum which also maintain a fixed position. Among the dark lines, M. Fraunhofer 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 as any of the physical sciences. These lines are designated by the letters of the alphabet, beginning with B, which is in the red near the end of the spectrum; c is farther advanced in the red; D is in the orange; E, in the green; F, in the blue; G, in the indigo; and н, in the violet. By means of these fixed points, M. Fraunhofer has ascertained from prismatic observation the refrangibility of seven of the principal rays in each of ten different substances solid and liquid. The refraction increased in all from the red to the violet end of the spectrum; but so irregularly for each ray and in each medium, that no law could be discovered. The rays that are wanting in the solar spectrum which occasion the dark lines,

were supposed to be 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 starlight 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. But the annular eclipse which happened on the 15th of May, 1836, afforded Professor Forbes the means of proving that the dark lines in question cannot be attributed to the absorption of the solar atmosphere; they were neither broader nor more numerous in the spectrum formed during that phenomenon than at any other time, though the rays came only from the circumference of the sun's disc, and consequently had to traverse a greater depth of his atmosphere. We are therefore still ignorant of the cause of these rayless bands.

A sunbeam received on a screen, after passing through a small round hole in a window-shutter, appears like a round white spot; but when a prism is interposed, the beam no longer occupies the same space. It is separated into the prismatic colors, and spread over a line of considerable length, while its breadth remains the same with that of the white spot. The act of spreading or separation is called the dispersion of the colored rays. Dispersion always takes place in the plane of refraction, and is greater as the angle of incidence is greater. It varies inversely as the length of a wave of light, and directly as its velocity: hence toward the blue end of the spectrum, where the undulations of the rays are least, the dispersion is greatest. Substances have very different dispersive powers; that is to say the spectra formed by two equal prisms of different substances under precisely the same circumstances, are of different lengths. Thus, if a prism of flint glass and one of crown glass of equal refracting angles be presented to two rays of white light at equal angles, it will be found, that the space over which the colored rays are dispersed by the

flint glass is much greater than the space occupied by that produced by the crown glass; and as the quantity of dispersion depends upon the refracting angle of the prism, the angles of the two prisms may be made such, that when the prisms are placed close together with their edges turned opposite ways, they will exactly oppose each other's action, and will refract the colored rays equally but in contrary directions, so that an exact compensation will be effected, and the light will be refracted without color (N. 191). The achromatic telescope is constructed on this principle. It consists of a tube with an object glass or lens at one end to bring the rays to a focus and form an image of the distant object, and a magnifying glass at the other end to view the image thus formed. Now it is found that the object-glass, instead of making the rays converge to one point, disperses them, and gives a confused and colored image: but by constructing it of two lenses in contact, one of flint and the other of crown glass of certain forms and proportions, the dispersion is counteracted, and a perfectly well defined and colorless image of the object is formed (N. 192). It was thought to be impossible to produce refraction without color, till Mr. Hall, a gentleman of Worcestershire, constructed a telescope on this principle in the year 1733; and twenty-five years afterward, the achromatic telescope was brought to perfection by Mr. Dollond, a celebrated optician in London.

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. 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 appears, 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, thé accidental is then called the complementary color, because any two colors are said to be complementary to one another which produce white when combined.

From recent experiments by M. Plateau of Brussels, 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; and in every case where the real colors produce white by their combination, the accidental colors of the same tints produce black. 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. M. Plateau attributes this phenomenon to a reaction of the retina after being excited by direct vision, so that the accidental impression is of an opposite nature to the corresponding direct impression. He conceives, that when the eye is excited by being fixed for a time on a colored object, and then withdrawn from the excitement, that it endeavors to return to its state of repose, but in so doing that it passes this point and spontaneously assumes an opposite condition, like a spring, which, bent in one direction, in returning to its state of rest bends as much the contrary way. The accidental image thus results from a particular modification of the organ of sight, in virtue of which it spontaneously gives us a new sensation after it has been excited by direct vision. If the prevailing impression be a very strong white light, its accidental image is not black, but a variety of colors in succession. According to M. Plateau, the retina offers a resistance to the action of light, which increases with the duration of this action; whence, after looking intently at an object for a long time, it appears to decrease in brilliancy. The im

agination has a powerful influence on our optical impressions, and has been known to revive the images of highly luminous objects months, and even years, afterward.

SECTION XX.

Interference of Light-Undulatory Theory of Light-Propagation of Light -Newton's Rings-Measurement of the Length of the Waves of Light, and of the Frequency of the Vibrations of Ether for each Color-Newton's Scale of Colors-Diffraction of Light-Sir John Herschel's Theory of the Absorption of Light-Refraction and Reflection of Light.

NEWTON and most of his immediate successors imagined light to be a material substance, emitted by all selfluminous 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 explained by this theory; it is, however, totally inadequate to account for the following circumstances.

When two equal rays of red light, proceeding from two luminous points, fall upon a sheet of white paper in a dark room, they 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. The same

effect will take place if the difference in the lengths be twice, 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 11, 2, 3, &c. part, the one light will entirely extinguish the other, and will produce absolute darkness on the paper where the united beams 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 be