CHAPTER V. PHENOMENA DEPENDANT ON THE DISTRIBUTION AND CONDITIONS OF LIGHT. In this chapter we shall endeavour to give a general description of the most remarkable terrestrial phenomena which result from the conditions of light, and to explain the laws by which they are governed. It is not our intention to make any extended remarks on the opinions which have been entertained concerning the nature of light as a physical agent, a subject which has always been involved in doubt, or rather in inexplicable mystery; nor can we refer minutely to those physico-mathematical laws which govern its intensity, distribution, and colour, though it will be necessary to refer to some of these for the explanation of the phenomena that properly come under consideration. We will admit the existence of a principle called light, and that the organ it is adapted to affect is the most extensive and accurate of all the mediums of sense, the best calculated to add to our pleasures, and to increase our intellectual resources. From these admitted facts we will endeavour to trace the peculiarities which, under various circumstances, produce the different luminous appearances around and above us. Indeed, no science is more worthy of investigation than the science of optics, although it is generally less understood than many others. By a knowledge of the properties of light, man has received a marvellous increase to the extent of his vision;-things too distant or too near for the natural range of his sight, are brought within the compass of his observation by a mechanical application of this science; and, in a limited acceptation, he possesses the power of giving sight to the blind, by relieving the misfortunes of some, and the infirmities of others. PRODUCTION OF LIGHT. Light is produced under various circumstances, and by means which we cannot very readily explain. It has been : supposed by some authors that light is capable of entering into all bodies, and that it is, in fact, a component part of them and, whatever objections may be made to this statement, there can be no doubt that many substances have the property of emitting light in the dark, after exposure to the solar rays. Thus, sulphate of barytes, Baldwin's phosphorus, and other compounds, become phosphoric after exposure to light; and it has been stated that a diamond, which, after being placed in the solar rays, was immediately covered with black wax, shone for several years after the wax was removed. But none of these substances emit light of the same colour as that which falls upon them, a circumstance that may be attributed to a partial absorption. There are also some insects that seem to have the power of absorbing light, and its emission is in some measure under their own control. The glow-worm, which is the Lampyris noctiluca of Linnæus, is common in some parts of England, and shines with a strong sulphur-coloured light. This property, however, seems to be only possessed by the female, which is a wingless insect, and resembles in form the larva of a beetle. There is, however, a species of Elater, a native of the West India islands, that is still more strongly endowed with this property. This insect is not more than an inch long; but the light, which it emits from two transparent eyelike tubercles placed upon the thorax, is so intense, that the smallest print may be read by moving one of these insects over the page. It is said that the luminous insects derive their peculiar property from a liquor which they secrete, and that they lose their brilliancy if it be suffered to dry upon the hand. It is not therefore true that the luminous appearance is altogether voluntary, and this is still farther proved by its continuance for some time after the death of the insect. Certain fishes also possess the same property. The Pholas and the Lampyris have been long known to emit phosphorescent scintillations, and the Roman epicures were accustomed to darken their apartments when feasting on them, that they might gratify two senses at the same time. There is another luminous appearance, commonly called Will-with-the-wisp, or Jack-with-the-lantern, which is supposed by the uninformed peasantry to be the visible representation of an evil spirit, that delights to delude the benighted traveller, conducting him by its light into bogs and morasses, and then leaving him to extricate himself as he can rom his perilous situation "A wand'ring fire, Hovering and blazing with delusive light, Milton. Mr. Bradley supposed this appearance to be occasioned by a swarm of luminous insects, and Mr. Ray was of the same opinion. It is generally produced by the presence of phosphorated hydrogen gas, which inflames at the common temperature of the atmosphere, but it may be sometimes occasioned by a strongly electrified animal vapour. And here it may be observed, that the incipient decomposition of animal and vegetable substances is generally attended with luminous appearances, and this is particularly the case with fishes. Light is also given out during combustion, and it may also be produced by a variety of mechanical means. When two pieces of lump sugar, agate, or quartz, are rubbed violently together, a vivid yellow light is produced. The New Zealanders and other savages produce fire by the friction of two smooth pieces of wood, and forests are inflamed by the same means when two branches or trunks of trees, agitated by the wind, are violently rubbed together. Under all these circumstances light is produced, and its appearance would almost lead the observer to imagine that it is a constituent part of bodies. Some philosophers do believe it to be a component part of combustible bodies, an opinion which seems to be justified by the experiments of Dieman and Pacts. These chymists exposed to a high temperature a mixture of sulphur and zinc, excluding every substance from which they might obtain oxygen. The two substances united without oxydation, and formed a sulphuret of zinc, and at the moment of combination gave out a vivid light. Without admitting light to be a constituent of combustible bodies, it is almost impossible to account for the variety of coloured flames produced by substances, and the principle of absorption would lead to the same result. There must, however, be some primary source from which all the luminous bodies on the earth's surface derive their property, and solar light is that source, a principle that pervades the system of which our little world is a member, emanating from, or influenced by, the orb of day, and flowing, as it were, from a centre to the very boundaries of its acknowledged attractive influence. But although this principle is so widely distributed, and exerts so important an influence upon living animals and vegetables, we can only form conjectures upon its nature, our observations and experiments being confined to its influence on bodies, and the laws by which it is governed under particular circumstances. NATURE OF LIGHT. Two theories have been proposed to explain the nature of light, and it may be necessary to mention them. One of these was advanced by Newton, who supposes light to be a material substance, emitted in extremely minute particles from all luminous bodies; these particles, impinging upon the retina of the eye, produce the sensation of light. This theory readily explains the greater number of luminous appearances, but there are some of which it does not offer a very ready explanation. For a long time it was universally adopted, and is still acknowledged by many philosophers, though it has been greatly superseded by the doctrine of undulations. It has been long known that sound is occasioned by undulations excited in atmospheric air, or some other conducting medium, by a vibrating body; and, in consequence of this, probably, it was suggested by Des Cartes that light also might be produced by the undulations excited in an ether of extreme rarity filling all space. This hypothesis, however, is due to Aristotle, who supposed light to result from the motion of a pure, subtile, homogeneous medium, or ether, excited by the solar rays. But it owes its present form to the successful labours of Euler, Young, and Fresnel. According to this theory, the elastic medium pervades not only all space, but all substances, and is excited in the solar system by some undefined action of the sun. Professor Airy has supported the hypothesis with the most refined analysis; and if we did not know that many things pronounced mathematically certain have been found experimentally false, we could not withhold our belief in the doctrine; but it is always difficult to speculate upon the nature of occult agents, and it may be doubted whether the question proposed in Milton's beautiful apostrophe to light has been or ever will be answered: "Hail, holy light! offspring of heaven, first-born, Bright effluence of bright essence increate- The rising world of waters dark and deep." GENERAL FACTS. The elementary principles of a science are sometimes so distinctly exhibited in natural appearances, that the most casual observer may acquaint himself with them, though he may not perceive their connexion or importance. But in other instances they can only be ascertained by close observation or by minute analysis. Without an acquaintance with these principles, the student only accumulates difficulties as he increases his knowledge; it may therefore be necessary to make a few preliminary observations in relation to the nature, habit, and character of light under ordinary circumstances, when uninfluenced by the disturbing forces to which it is subject. 1. Light requires time for its propagation from the luminous to the enlightened body. On account of the immense velocity with which light moves, this fact could never have been ascertained by the observation of terrestrial phenomena; for although it is true that the flash of a gun or the light of a beacon may be seen by a person one mile distant when it is invisible to a person at the distance of two miles, yet, if it were possible to measure the hundredths of a second with the same accuracy as we measure seconds, we could not ascertain the fact; it is only by reference to the celestial bodies that we can discover the velocity of light. Aristotle taught the instantaneous progression of light; and Chrysippus, the stoic, who was the successor of Zeno, says that its transference is like to the motion of a long rod, which moves equably through its whole length when pushed at one end. For a long series of ages philosophers acknowledged and taught this error, and M. Roemer, a native of Jutland, discovered the truth when making observations on the satellites of Jupiter. When the earth is between Jupiter and the sun, eclipses of the satellites of that planet happen eight minutes thirteen seconds earlier than they should do according to the calculations of astronomers; and when the sun is between |