most brilliant when viewed in light of their own colour. 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 coloured 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 coloured 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 colour of red. All tints have their accidental colours:-thus the accidental colour of orange is blue; that of yellow is indigo; of green, reddishwhite; of blue, orange-red; of violet, yellow; and of white, black; and vice versa. When the direct and accidental colours are of the same intensity, the accidental is then called the complementary colour, because any two colours are said to be complementary to one another which produce white when combined.

Recent experiments by Plateau of Brussels prove that direct and accidental colours differ essentially. From these it appears that two complementary colours 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 colours 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 colour 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 colours in succession. With a little attention it will generally be found that, whenever the eye is affected by one prevailing colour, it sees at the same time the accidental colour, 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.

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. The same effect will take place if the difference in their 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 1, 2, 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 colours 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 coloured 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 colours, arising from the superposition of systems of alternate black lines and lines of each homogeneous colour. That the alternation of black lines and coloured 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 another. 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, it is impossible not to be struck with the perfect similarity between the interferences of small undulations of air and water and the preceding phenomena. The analogy is indeed so perfect, that philosophers of the highest authority concur in the supposition that