Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/604

584 LIGHT Bunsen photo meter. Wheat- stone s. Difficul ties of photo metry. Chemical photo metry. Velocity of light. Rower s nietlioil. lit up by the other alone ; and here again the amounts of light given out by the sources are as the squares of their distances from the screen when the shadows are equally intense. The shadow-casting object should be near the screen, so as to avoid penumbra as much as possible ; yet not too near, so that the shadows may not overlap. Bunsen has recently suggested the very simple expedient of making a grease-spot on white paper for photometric purposes. When the paper is equally illuminated from both sides, the grease-spot cannot be seen except by very close inspection. In using this photometer, the sources are placed in one line with the grease-spot, which lies between them and can be moved towards one or other. To make the most accurate determinations with this arrangement the adjustment should first be made from the side on which one source lies, then the screen turned round and the adjustment made from the side of the other source, in both cases, therefore, from the same side of the paper screen. Take the mean of these positions (which are usually very close together), and the amounts of light are as the squares of the distances of the sources from this point. Wheatstone suggested a hollow glass bead, silvered internally, and made to describe very rapidly a closed path, for use as a photometer. When it is placed between two sources, we see two parallel curves of reflected light, one due to each source. Make these, by trial, equally bright ; and the amounts of light from the sources are, again, as the squares of the distances. These simple forms of apparatus give results which are fairly accurate, so long at least as the qualities of the light furnished by the two sources are nearly the same. But, when we endeavour to compare differently coloured lights, the result is by no means so satisfactory. In fact, we cannot well define equality of illumination when the lights are of different qualities. In the undulatory theory, no doubt, we can distinctly define the intensity of any form of radiation. But the definition is a purely dynamical one, and has not necessarily any connexion with what we usually mean by intensity, viz., the amount of effect produced upon the nerves of the retina. Thus the theoretical intensities of a given violet and a given red source may be equal, while one may appear to the eye very much brighter than the other. Think, for instance, of a colour-blind person, who might, under conceivable circumstances, be unable to see the red at all. We are all as it were colour-blind as far as regards radiations whose wave-lengths are longer or shorter than those included in the range of the ordinary solar spectrum. Other modes of measuring the intensity of light usually depend upon more recondite physical principles, such as, for instance, the amounts of chemical action of certain kinds which can be produced by an exposure of a given duration to the light from a particular source. But all have the same grand defect as the simpler processes, they are not adapted to the comparison of sources giving different qualities of light. And those last mentioned are liable to another source of error, viz., the action of radiations which are not called light, only because they are not visible to the eye ; for in all other respects they closely resemble light, and are often more active than it is in pro ducing chemical changes. VELOCITY OF LIGHT. Light moves with a velocity of nearly 180,000 miles per second. Of this we have four distinct kinds of proof, on each of which depends a method which is capable of giving pretty accurate results. 1. Kumtr s Method. By this the finite velocity of light was discovered in 1G7G. Suppose, to illustrate this, that at a certain place a cannon is fired precisely at intervals of an lunr while the weather is perfectly calm. A person provided with an accurate watch travels about in the suv- rounding district. When he first hears the cannon let him note the time by his watch, then on account of the non- instantaneous propagation of sound, if at the next discharge he be nearer the gun than before, the report will arrive at his ear before the hour s interval has elapsed ; if he be farther from the gun, the interval between the discharges will appear longer than an hour; and the number of seconds of defect or excess will evidently represent tho time employed by sound in travelling over a space equal to the difference of his distances from the gun at the suc cessive observations. Now the satellites of Jupiter are subject like our moon, only much more frequently to eclipse, and the interval between two successive eclipses can easily be observed. The almost sudden extinction of the light is a phenomenon similar to the discharge of the gun ; and, if light take time to move from one place to another, we should find the interval between successive eclipses too short when we are approaching Jupiter, too long when we are receding from him. Such was found to be the case by Eomer ; and he also found that the shortening or lengthening of the interval depended upon the rate at which the earth was approaching to or receding from Jupiter. The inevitable conclusion from these facts is that light is propagated with finite velocity. Homer calculated from them that light takes about 165 to cross the earth s orbit. The exact velocity deduced by this method is, after making all corrections, and assuming the most probable value of the solar parallax, about 186,500 miles per second. 2. Bradley s Method. This depends on the aberration o/ Bradley light, discovered by Bradley in 1728. When in a calm method. rainy day one stands still he holds his umbrella vertical in order to protect himself. If he walk he requires to hold it forwards, and more inclined the faster he walks. In other words, to a person walking the rain does not appear to come in the same direction as to a person standing still. 1 Now the earth s velocity in its orbit is a very large quan tity, some 18| miles per second, or about -nny-<jtfth of that of light. Hence the light from a star does not appear to come in the proper direction unless the earth be moving exactly to or from the star, and, as the direction of the earth s motion is continually changing, so the directions in which different stars are seen are always changing, and thus this phenomenon, called the " aberration of light," proves at once the finite velocity of light and the earth s motion round the sun. As an additional illustration of the phenomenon, suppose a bullet fired through a railway carriage, in a direction perpendicular to the sides of the carriage. If the carnage be standing still, the bullet will make holes in the sides, the line joining which is perpendicular to the length of the carriage; if it be in motion, then the second side of the carriage will have moved through a certain space during the interval occupied by the bullet iu passing from side to side, and thus tha line joining the holes in the sides (i.e., the line pursued by the bullet relatively to the carriage), will be inclined at an angle greater than a right angle to the direction of the train s motion. It is evident that the path apparently described by each star during a year, in consequence of aberration, will be found by laying off from the star lines which bear the same ratio to the star s distance as the velocity of the earth does to that of light, their directions being always the same as that of the earth s motion at every instant. This is precisely the definition of the HODOGRAPH (q.v.} of the earth s orbit. Hence, on account of the finite velocity of 1 In fact, to estimate the relative direction and velocity of two moving bodies we must subtract the vector velocity of the first from that of the second.