The shadow pantomimic action of living figures visible on a transparent disc with this strong light, and first introduced by the author at the Polytechnic, has gone the round of nearly all the exhibitions and theatres in London and New York.

There still remains, however, something new and amusing even in this hackneyed branch of light. Mr. Walker, jun., constructed a very simple and ingenious piece of mechanism, and giving it the outline of a human figure, produced a good imitation of the bold feats performed by Monsieur Blondin on the high rope. The shadow of the figure only was projected on to the disc by the lime-light, and it simulated all the usual movements, such as standing, walking, dancing, and sitting astride the rope. Indeed it did rather more than the living prototype, for the figure stood on its head, and threw the most unnatural but highly-amusing sommersaults. (Figs. 10, 11, 12, pp. 12, 13.)

THE DIFFUSION OF LIGHT.

A luminous object evolves light from every visible point of its surface, and if a single point of light were placed in the centre of a hollow globe, every portion of the internal area would be equally illuminated.

FIG. 13.-A Flame in the centre of a circle, throwing out rays in
every direction, like the spokes of a wheel.

Owing to the manner in which light is distributed and transmitted in straight lines diverging from each other, its intensity diminishes as the square of its distance from the luminous source increases, and it is on this principle that the instruments called photometers, or light-measurers, are constructed.

A scale of 20 ft. in length, divided into feet and inches, may be used in conjunction with a box somewhat like a stereoscope, containing two mirrors placed at an angle of 45°, and reflecting the rays from the two sources of light which are to be confronted with each other. A candle, one of six to the

pound, and burning so many grains per minute, is fixed in a nozzle, which slides on the scale. The box, which may also slide or be fixed in the centre of the scale, reflects on one side the light from the lamp or gas-burner which is being tested, on the other it reflects the light of the candle. The experiment may be conducted either by placing the lamp and the candle at opposite ends of the scale, and moving the box with the reflectors until the two spots of light are equal; or, the box being fixed in the centre, and the lamp under examination placed at one end of the scale, the candle may be moved towards the box till the lights are equal, the respective distances from the box being then squared, and the greater number divided by the less, will give the quotient which represents the illuminating power of the lamp as compared with the candle.

FIGS. 14 and 15.-Ritchie's Photometer.

Section of the box containing the mirrors A B, A C, openings DO, EO, to admit the light which is reflected from the mirrors on to two circular apertures P P, covered with oiled paper, which are seen and compared when looked at from the top at T T. The arrows indicate the direction of the rays from the lamp, and L, the wax candle w c. Example: the distance of the lights from the box being respectively 12 ft. and 3 ft. — 12 X 12 =144 ÷ 3 X 39. Quotient, 16.

In the practice of photometry the standard used is a candle defined by Act of Parliament "as a sperm candle of six to the pound, burning at the rate of 120 grains per hour." This standard would be a very simple one if every candle could be made alike, but it unfortunately happens that the composition. and the wick are not always the same, and as important experiments have to be made in various parts of the United Kingdom, it becomes difficult to assimilate and compare them with each other. All authorities on this question have condemned the use of test candles. The credit is due to Mr. Crookes, the editor of the "Chemical News," of devising a standard test lamp-wick and combustible fluid which could be made in every part of the civilized world, and of inventing an improved photometer, in which the phenomena of polarized light are employed. The following is the inventor's description of the apparatus and materials used, commencing with the lamp and its fuel:* "Alcohol of sp. gr. 0.805, and pure benzol boiling at 81° C., are mixed together in the proportion of 5 volumes of the former and I of the latter. This burning fluid can be accurately imitated from description at any future time and in any country, and if a lamp could be devised equally simple and invariable, the light which it would yield would, it is presumed, be invariable. This difficulty the writer has attempted to overcome in the following manner.

"Chemical News," July 17th, 1868.

"A glass lamp is taken of about 2 ounces capacity, the aperture in the neck being o'25 inch diameter; another aperture at the side allows the liquid fuel to be introduced, and, by a well-known laboratory device, the level of the fluid in the lamp can be kept uniform. The wick-holder consists of a platinum tube 181 in. long, and o125 in. internal diameter. The bottom of this is closed with a flat plug of platinum, apertures being left in the sides to allow free access of spirit. A small platinum cup 0.5 in. diameter and I in. deep is soldered round the outside of the tube o'5 in. from the top, answering the threefold purpose of keeping the wick-holder at a proper height in the lamp, preventing evaporation of the liquid, and keeping out dust. The wick consists of 52 pieces of hard-drawn platinum wire, each o'or in. diameter and 2 in. long, perfectly straight, and tightly pushed down into the platinum-holder until only o'I in. projects above the tube. The height of the burning fluid in the lamp must be sufficient to cover the bottom of the wick-holder; it answers best to keep it always at the uniform distance of 175 in. from the top of the platinum wick; a slight variation of level, however, has not been found to influence the light to an extent appreciable by our present means of photometry. The lamp having the reservoir of spirit thus arranged, the platinum wires parallel, and their projecting ends level, a light is applied, and the flame instantly appears, forming a perfectly shaped cone 125 in. in height, the point of maximum brilliancy being o'56 in. from the top of the wick. The extremity of the flame is perfectly sharp, without any tendency to smoke; without flicker or movement of any kind; it burns, when protected from currents of air, at a uniform rate of 136 gr. of liquid per hour. The temperature should be about 60° F., although moderate variations on ither side exert no perceptible influence. Bearing in mind Dr. Franklin's observations on the direct increase in the light of a candle with the atmospheric pressure, accurate observations ought only to be taken at one height of the barometer To avoid the inconvenience and delay which this would occasion, a table of corrections should be constructed for each o‘I variation of barometric pressure.

"There is no doubt that this flame is very much more uniform than that of the sperm candle sold for photometric purposes. Tested against a candle, considerable variations in relative illuminating power have been observed; but on placing two of these lamps in opposition, no such variations have been detected. The same candle has been used, and the experiments have been repeated at wide intervals, using all usual precautions to ensure uniformity." The results are thus shown to be due to variations in the candle, and not in the lamp.

In Arago's "Astronomy," the author describes his photometer in the following words:

"I have constructed an apparatus by means of which, upon operating with the polarized image of a star, we can succeed in attenuating its intensity by degrees exactly calculable after a law which I have demonstrated.'" It is difficult to obtain an exact idea of this instrument from the description given; but from the drawings it would appear to be exceedingly complicated, and to be different in principle and construction from the one now about to be described. The present photometer has this in common with that of Arago, as well as with those described in 1853 by Bernard,* and in 1854 by Babinet,+

"Comptes Rendus," April 25, 1853.

+"Proceedings of the British Association," Liverpool Meeting, 1854.

that the phenomena of polarized light are used for effecting the desired end. But it is believed that the present arrangement is quite new, and it certainly appears to answer the purpose in a way which leaves little to be desired. The instrument will be better understood if the principles on which it is based are first described.

+

+

C

H

"Fig. 16 shows a plan of the arrangement of parts, not drawn to scale, and only to be regarded as an outline sketch to assist in the comprehension of general principles. Let D represent a source of light. This may be a white disc of porcelain or paper illuminated by any artificial or natural light. C represents a similar white disc likewise illuminated. It is required to compare the photometric intensities of D and C. (It is necessary that neither D nor C should contain any polarized light, but that the light coming from them, represented on each disc by the two lines at right angles to each other, forming a cross, should be entirely unpolarized.) Let H represent a double-refracting achromatic prism of Iceland spar; this will resolve the disc D into two discs, d and d', polarized in opposite directions; the plane of d being, we will assume, vertical, and that of a horizontal. The prism H will likewise give two images of the disc C; the image c being polarized horizontally, and c' vertically. The size of the discs D, C, and the separating power of the prism H are to be so arranged that the vertically polarized image d, and the horizontally polarized image c, exactly overlap each other, forming, as shown in the figure, one compound disc, cd, built up of half the light from D and half that from c.

"The measure of the amount of free polarization present in the disc c d, will give the relative photometric intensities of D and C.

+

d

g

D

FIG. 16.

I

"The letter I represents a diaphragm with a circular hole in the centre, just large enough to allow the compound disc c d to be seen, but cutting off from view the side discs cd'. In front of the aperture in I is placed a piece of selenite of appropriate thickness for it to give a strongly-contrasting red and green image under the influence of polarized light. K is a doubly-refracting prism, similar in all respects to H, placed at such a distance from the aperture in I that the two discs into which I appears to be split up are separated from each other, as at g D. If the disc c d contains no polarized light, the images gr will be white, consisting of oppositely polarized rays of white light; but if there is a trace of polarized light in c d, the two discs gr will be coloured complementarily, the contrast between the green and red being stronger in proportion to the quantity of polarized light in c d.

"The action of this arrangement will be readily evident. Let it be supposed in the first place that the two sources of light, D and C, are exactly equal. They will each be divided by H into two discs, d' d and c c', and the two polarized rays of which c d is compounded will also be absolutely equal in intensity, and will neutralize each other and form common light, no trace of free polarization being present. In this case the two discs of light g D will be colourless. Let it now be supposed that one source of light (D for instance)

is stronger than the other (C). It follows that the two images d'd will be more luminous than the two images c c', and that the vertically polarized ray d will be stronger than the horizontally polarized ray c. The compound disc cd will therefore shine with partially polarized light, the amount of free polarization being in exact ratio with the photometric intensity of D over C. "In this case the image of the selenite plate in front of the aperture I will be divided by K into a red and a green disc.

"Fig. 17 shows the instrument fitted up. A is the eye-piece (shown in enlarged section at Fig. 3). GB is a brass tube, blacked inside, having a piece, shown separate at D C, slipping into the end B. The sloping sides, D B, B C, are covered with a white reflecting surface (white paper or finely ground porcelain), so that when D C is pushed into the end B, one white surface, D B, may be illuminated (as in Fig. 17) by the candle, and the other surface, B C, by the lamp. If the eye-piece A is removed, the observer, looking down the tube G B, will see at the end a luminous white disc divided vertically into two parts, one half being illuminated by the candle E, and the other half by the lamp F. By moving the candle E, for instance, along the scale, the illumination of the half D B can be varied at will, the illumination of the other half remaining stationary.

"The eye-piece A (shown enlarged at Fig. 18) will be understood by reference to Fig. 16, the same letters representing similar parts. At L is a lens to collect the rays from B D C, Fig. 17), and throw the image into the proper part of the tube. At M is another lens, so adjusted as to give a sharp image of the two discs into which I is divided by the prism K. The part N is an adaptation of Arago's polarimeter; it consists of a series of thin plates of glass capable of moving round the axis of the tube, and furnished with a pointer and graduated arc. By means of this pile it is possible to partially polarize the rays coming from the illuminated discs in one or the other direction, and thus bring to the neutral state the partially polarized beam c d (Fig. 16), so as to get the images g D free from colour. It is so adjusted that when at the zero point it produces an equal effect on both discs.