LIGHT 579 Colour colours. lu colour-blindness one or more of these organs of blind- sense is wanting, or imperfect. The most common form, ess> Daltonism, depends on the absence of the red sense. Great additions to our knowledge of this subject, if only in con firmation of results already deduced from theory, have been obtained in the last few years by Holmgren l; who has experimented on two persons, each of whom was found to have one colour-blind eye, the other being nearly normal. In this way was obtained, what could otherwise have been matter of conjecture only, a description of colour-blind vision in terms of (at least approximately) normal vision. Duration Finally, the sensation of sight is not limited to the of visual duration of the mechanical action on the eye. It is known that we do not see a sudden flash (an electric spark for instance) until a measurable, though very short, period has elapsed. This depends on the rate at which an excitation is propagated along the optic nerve. But the familiar experiment of whirling a red-hot stick hi a dark room shows that the sensation of sight lasts for a short period after the mechanical action which produced it has ceased. This period is probably different for different eyes, and for different amounts of excitement even in the same eye. (If the light be very intense the effect lasts much longer, but completely changes its character.) For our present purpose it may be assumed that the dura tion is somewhere about yth of a second. Thus, if the end of the red-hot stick describes a circle once in yth of a second, we see the complete circle; if in a longer period, we only see at once such a part of it as was described in Intensity -}th of a second. Connected with this is the remarkable result obtained experimentally by Swan, 2 that the amount on^lura- of sensation is, for flashes of short duration, directly pro- tion of portional, not only to the brightness of the flash, but also exciting to its duration. A flash which lasts for y^th of a second cause. produces the full effect on the eye; but an electric spark, as a flash of lightning, which certainly does not endure for more than T7znyzn5TT t h f a second, produces at most only Tornnjo-th ^ the effect it would produce if it lasted j^th of a second. On this short duration of visual im pressions depends the action of the thaumatrope, the ivheel of life, etc. By various kinds of machinery a succession of views of an object in different positions or forms is presented to the eye, each for a brief interval. The result is that we fancy we see one and the same object going through a species of continuous motion, or of change of form, which would present it to the eye in these succes sive positions or forms. Thus, a tadpole may be repre sented as wriggling about, or as developing continuously into a frog, &c. SOURCES OF LIGHT. This subject will be fully treated in other parts of this work under various heads : from the purely scientific point of view under RADIATION; from the more practical side under LIGHTING (ELECTRIC), &c. For our present purpose a very brief summary of the question will suffice; and we do not require to investigate the process by which, in any case, the light is produced. Incan- 1. The main source of light is incandescence. (It is descence. usually understood that to be incandescent a body must be at a high temperature.) This may be due to any of a number of causes, such as the following : (a) The Potential Energy of Gravitation of Scattered Fragments of Matter. When these fall together, as in the formation of the sun and stars, heat enough is generated by impact to render the whole vividly incandescent. It is probable that the light of nebula, and the praper light of comets, is due to this cause. The proximate cause, in all these cases, is the kinetic energy of the fragments 1 Proc. Roy. Soc., Jan. 1881. 2 Trans. Roy. Soc. Edin., 1849, 1861. before impact. To this class, therefore, can be reduced the light given out when a target is struck by a cannon shot. (b) The Kinetic Energy of Current Electricity or of an Electric Discharge. Here we have lightning, the electric light, and probably also the light of the aurora. (c) The Potential Energy of CJiemical Affinity. The lime-light, gas-light, candle and lamp-light, fire-light, the magnesium light, &c.; also phosphorus, dead fish (?), &c., glowing in the dark. (d) Friction, as in the trains of sparks from a grindstone or brake; though here, in general, chemical affinity also has a share. (e) Sudden great Compression of a Gas, as of air by meteoric stones and falling stars. 2. Another very curious source, not (so far as is known) reducible to incandescence, is the giving out (usually in an altered form] of light previously absorbed: fluorescence, phosphorescence, &c. 3. A third source is physiological : fire-flies, glow worms, Medusa?, dead fish (?), etc., the eye of a cat. Any not black and not transparent body, exposed to any of these sources of light, becomes in its turn what may for our purpose also be treated as a source. As will be shown in EADIATION, the only bodies which, when incandescent, give every constituent of white light are bodies which are black in the sense of absorbing each and every ray which falls upon them. Such bodies are not necessarily solids though the best examples we have of them are lamp-black, and (somewhat less perfect) char coal and gas-coke. Newton s speculations on these subjects, taken from the " Queries " at the end of his Optics, give an exceedingly interesting sketch of the state of this subject in his time. We quote a few of the more curious. There is a strange admixture of errors, but a still more strange anticipation of some of the most important of modern discoveries. "Query 6. Do not Black bodies conceive heat more easily from light than those of other colours do, by reason that the light falling on them is not reflected outwards : but enters the bodies, and is often reflected and refracted within them, until it be stifled and lost? " Query 8. Do not all Fixed bodies, when heated beyond a certain degree, emit light and shine; and is not this emission performed by the vibrating motions of their Parts. And do not all bodies, which abound with Terrestrial parts, and especially with Sulphureous ones, emit light, as often as those Parts are sufficiently agitated; whether that agitation be made by heat, or by friction, or percussion. or putrefaction, or by any vital motion, or any other cause? . . . . Query 9. Is not Fire a body heated so hot, as to emit light copiously? For what else is a red hot iron than fire? And what else is a burning coal than red hot wood? "Query 10. Is not Flame a vapour, fume or exhalation heated red hot, that is, so hot as to shine? For bodies do not flame without emitting a copious fume, and this fume burns in the flame. The Ignis Fatuus is a vapour shining without heat; and is there not the same difference between this vapour and flame, as between rotted wood shining without heat and burning coals of fire? In distilling hot spirits, if the head of the still be taken off, the vapour, which ascends out of the still, will take fire at the flame of a candle, and turn into flame, and the flame will run along the vapour from the candle to the still. Some bodies heated by motion or fermentation, if the heat grow intense, fume copiously; and if the heat be great enough, the fumes will shine, and become flame. Metals in fusion do not flame for want of a copious fume, except spelter, which fumes copiously, and thereby flames. All flaming bodies, as oil, tallow, wax, wood, fossil coals, pitch, sul phur, by flaming waste and vanish into burning smoke; which smoke, if the flame be put out, is very thick and visible, and some times smells strongly, but in the flame loses its smell by burning; and, according to the nature of the smoke, the flame is of several colours; as that of sulphur, blue; that of copper opened with sublimate, green; that of tallow, yellow; that of camphire, white. Smoke passing through flame cannot but grow red hot; and red hot smoke can have no other appearance than that of flame ..... " Query 11. Do not Great bodies conserve their heat the longest, their parts heating one another; and may not Great dense and Fixed bodies, when heated beyond a certain degree, emit light so Fluor- escence. Newton on
