LIGHT 603 light is cut off from the fluorescent body. Becquerel s ingenious phosphoroscope was invented for the purpose of inquiries of this kind. It consists essentially of a shallow drum, in whose ends two excentric holes, exactly opposite to one another, are cut. Inside it are fixed two equal metnl disks, attached perpendicularly to an axis, and divided into the same number of sectors, the alternate sectors of each being cut out. One of these disks is close to one end of the drum, the other to the opposite end, and the sectors are so arranged that, when the disks are made to rotate, the hole in one end is open while that in the other is closed, and vice versa. If the eye be placed near one hole, and a ray of sunlight be admitted by the other, it is obvious that while the sun shines on an object inside the drum the aperture next the eye is closed, and vice versa. If the disks be made to revolve with great velocity by means of a train of toothed wheels, the object will be presented to the eye almost instantly after it has been exposed to sunlight ; and these presentations succeed one another so rapidly as to produce a sense of continued vision. By means of this apparatus we can test with considerable accuracy the duration of the phenomenon after the light has been cut off. For such a purpose we require merely to know the number of sectors in the disks and the rate at which they are turned. To guard against deception by the persistence of impressions on the retina, the eye should not be directed fixedly on the object, but should be kept travelling slowly round the position in which it is seen to lie. Uranium glass shows, with rapid turning, nearly as vivid an effect as when exposed to continuous light, but fades rapidly when the speed of the rotation falls off. A pinkish kind of ruby, exposed to concentrated sunlight in the phosphoroscope, is seen to glow with a bright red like a piece of live coal. With very rapid turning, feeble fluorescence can be detected in a great many substances in which the ordinary methods will not show it. This is due in great measure to the fact that the phosphoroscope entirely does away with the scattered light, which in the ordinary mode of examining these substances overpowers their feeble fluorescence. i- What is correctly termed phosphorescence has nothing - to do with phosphorus (whose luminosity in the dark is 1CC- due to slow oxidation), but it is merely a species of fluorescence which lasts -for a much longer time after the excitation has ceased than does that just described. Pliny speaks of various gems which shine with a light of their own, and Albertus Magnus knew that the diamond becomes phosphorescent when moderately heated. But the first discovery of phosphorescent substances, such as are now so common, belongs to the early part of the 17th century. During that century the Bologna stone (sulphide of barium) and Homberg s phosphorus (chloride of calcium) were dis covered. Canton s phosphorus (sulphide of calcium) dates from 1768. To the substances mentioned may now be added sulphide of strontium. Any of these sulphides, which must be carefully preserved from the air in sealed glass tubes, appears brilliantly luminous when carried from sunlight into a dark room, and for a long time after presents the general aspect of a hot body cooling. The rays which excite their luminosity are (as with the generality of fluorescent bodies) those of higher refrangibilities ; but the colours of the phosphorescent light are of the most varied kind, even in specimens of almost precisely the same chemical composition, but prepared at different times. The causes of this strange diversity are as yet quite unguessed at ; but the property has been taken advantage of for uminous the production of what are called luminous paints. The t behaviour of these substances is one of the most singular phenomena in optics. How they manage to store up so large a supply of energy during a short exposure to bright light, and to dole it out continuously for so long a time and mainly in the form of light, is exceedingly puzzling, specially as no other physical or chemical change has yet been found to accompany the process. Another curious fact connected with their behaviour was discovered by Becquerel. He found that the less refrangible rays have in some cases the power of arresting the emission of light from these bodies when they have been previously excited by higher rays. The chemical effects of light will be treated under PHOTOGRAPHY, so far as they are connected with decom position. Its effects in causing combination, as of hydrogen and chlorine, have already been treated under CHEMISTRY. UNDULATOEY THEORY OF LIGHT. The explanation of the fundamental laws of Geometrical Optics by the wave-theory requires some preliminary remarks. As the subject will be more fully discussed in a special article, we confine ourselves to what is strictly necessary for the immediate purposes of the present article. (a) The essential characteristic of wave-motion is that a Wave- disturbance of some kind is handed on from one portion of motion, a solid or fluid mass to another. In certain cases only, this disturbance is unaltered in amount and in kind as it proceeds. (b) So far as light is concerned, the velocity with which Velocity each particular species of disturbance passes in any direction of propa- through a homogeneous isotropic medium is constant and is gat the same for all directions. When the medium is not homo geneous, the velocity may vary from point to point. If the medium be not isotropic, the velocity may depend upon the direction of propagation. Examples of each of these pecu liarities will be met with presently. (c) When two or more separate disturbances simultane- Inter- ously affect the same portion of a medium, the effect may ference. be very complex. But, in the case of light, it has been found that a geometrical (or rather kinematical) superposi tion or composition agrees, at least to the degree of accuracy of the experiments, with all the observed facts. This would be the case, as a dynamical result, if the distortions due to wave-motion were always, even for the most power ful light, exceedingly small. On this is based the whole doctrine of interference, Young s grandest contribution to the wave-theory (1801). (d) The disturbance at any point of a medium, at any Huy- instant, is that due to the superposition of all the disturb- S e s s ances which reached it at that instant from the various ^*"^ surrounding parts of the medium. This is (in a somewhat generalized form) what is commonly known as Huygens s principle, first enunciated in 1678. (e) The front of a wave is defined at any instant as the Wave- continuous locus of all portions of the medium which, at front. that instant, are equally and similarly distorted. The word continuous is inserted because, in oscillatory wave- motion, such as that of light, a large number of successive waves are exactly equal and similar to one another. Thus we have a series of wave-fronts following one another, which are not to be considered as parts of one wave-front. The distance between two successive fronts in which the distortions are similar, measured in the direction in which the light is travelling, is called the wave-length. (f) The colour of homogeneous light depends entirely on Colour, the period of a wave, i.e., on the time of passage from one wave-front to the next. This is obviously the same thing as the time of a complete vibration of any one particle of the medium whatever be the velocity of light in the medium, or the consequent wave-length. These being premised, let us take the propagation of homogeneous light from a luminous point in a homo-
