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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, 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.

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, 21, 31, &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

tween 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 (N. 193); or by admitting the sun's light into a dark room through a pin-hole about the fortieth of an inch in diameter, receiving the image on a sheet of white paper, and holding a slender wire in the light. Its shadow will be found to consist 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, such as 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. It 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 Now it is contrary to all our ideas of matter to suppose that two particles of it should annihilate one another under any circumstances whatever; while on the contrary, two opposing motions may, and it is impossible not to be struck with the perfect similarity between the interferences of small undulations of air or of water and the preceding phenomena. The analogy is indeed so perfect, that philosophers of the highest auare filled with an extremely rare, imponderable, and concur in the supposition that the celestial regions

another.

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highly elastic medium or ether, whose particles are capable of receiving the vibrations communicated to them by self-luminous bodies, and of transmitting them to the ⚫ptic nerves, so as to produce the sensation of light. The acceleration in the mean motion of Encke's comet, as well as of the comet discovered by M. Biela, renders the existence of such a medium almost certain. It is clear that in this hypothesis, the alternate stripes of light and darkness are entirely the effect of the interference of the undulations; for by actual measurement, the length of a wave of the mean red rays of the solar spectrum is equal to the 0.0000258th part of an inch; consequently, when the elevation of the waves combine, they produce double the intensity of light that each would do singly; and when half a wave combines with a whole, that is, when the hollow of one wave is filled up by the elevation of another, darkness is the result. At intermediate points between these extremes, the intensity of the light corresponds to intermediate differences in the lengths of the rays.

The theory of interferences is a particular case of the general mechanical law of the superposition of small motions; whence it appears that the disturbance of a particle of an elastic medium, produced by two coexistent undulations, is the sum of the disturbances which each undulation would produce separately; consequently, the particle will move in the diagonal of a parallelogram, whose sides are the two undulations. If, therefore, the two undulations agree in direction, or nearly so, the resulting motion will be very nearly equal to their sum, and in the same direction: if they nearly oppose one another, the resulting motion will be nearly equal to their difference; and if the undulations be equal and opposite, the resultant will be zero, and the particle will remain at rest.

The preceding experiments, and the inferences deduced from them, which have led to the establishment of the doctrine of the undulations of light, are the most splendid memorials of our illustrious countryman Dr. Thomas Young, though Huygens was the first to originate the idea.

It is supposed that the particles of luminous bodies

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