PHOTOMETER. An instrument for measuring the intensity of light, or of illumination.
The one which most usually goes under this name is the photometer in vented by the late Sir John Leslie. It is merely the differential thermometer of the same ingenious philosopher, having one of its balls diaphanous, and the other coated with China ink, or blown of deep black enamel ; and the _ whole covered a case of thin transparent glass, to &tend the balls from the disturbing influence of currents of air. The photometer has two general forms ; the one portable (fig. 1), in which the black ball is about an inch higher than the other, and bent forward to the same vertical line, or the axis of the translucent cylindrical ease ; and the other stationary (fig. 2), having both its balls of the same height, and reclining in opposite ways : the case being composed of a wide cylinder surmounted by the larger segment of a hollow glass sphere. The latter form of the instrument, though less coin modious, is better adapted for nice observations ; since, besides receiv ing the light more regularly, its balls, from being on the same level, are not liable to be any how disturbed in their indications by difibrent strata of unequal ly heated air.
The theory of this photometer de pends on the assumed principle that the intensity of light is proportional to the heat excited by its incidence on the black ball. When the instrument is ex posed to light, the rays which fall on the clear ball pass through it without suffer ing obstruction ; but those which strike the dark ball are stopped and absorbed at its surface, where, assuming a latent form, they act as heat, which, by ex panding the air within the ball, causes the liquid in the stem to descend. This heat will continue to accumulate till its farther increase comes to be counter acted by an opposite dispersion, caused by the rise of temperature which the hall has acquired. But, in still air, the rate of cooling is, within moderate limits, proportional to the excess of the temper ature of a given surface above that of the surrounding medium. Hence the space through which the colored liquid sinks in the stem will measure the mo mentary- impressions of light, or its actual intensity.
The graduation is entirely arbitrary, and may be regulated according to fancy or convenience. Leslie adopted the same scale of divisions as in the different ther mometer, ten degrees of which corres pond to one of the centigrade thermo meter. When the temperature of both balls is exactly the same, that is, when the instrument is excluded from light, the liquid in the stem next the colored ball stands at zero. In England the direct impression of the sun at noon, about the summer solstice, forces the li quid down to 90° or 100°. The greatest force of the solar beams, in the depth of winter, measure only about 25°. At the altitude of 3° above the horizon, the whole effect of the sun's rays does not ex ceed one degree. The indirect light of the sky at noon in the summer is from 30° to 40° ; in winter from 10° to 15°. Comparing the illuminating power of the solar rays with that of artificial lights, Leslie fbund the light emitted by the sun 12,000 times more powerful than that of a wax candle ; that is to say, if a portion of the luminous solar matter, rather less than half an inch in diameter, were trans mitted to our planet, it would throw forth a light equal to the effect of 12,000 candles.
A great objection to this instrument is, that the same quantity of light emitted by terrestrial bodies of different kinds is not always accompanied with the same degree of heat. Thus, phosphorus burns in oxygen gas with intense splendor, and yet gives out far less heat than he comparatively dull combustion of hydrogen in the same gas; and the pho tometer is more affected by a fire so dull ,hat not a single letter could be discerned n a well-printed page, than by the de cree of daylight by which the same page rould be read with pleasure and facility. PHOTOMETRY. The science which reats of the measurement of light. At empts to determine the relative inten tities of different lights were made at an :arly period in the history of experimen al science. For the purpose of compar ug the light of Sirius with that of the .un, the celebrated Huygens employed tube having a very small aperture at me end, into which was inserted ninute globular lens, which allowed only he 27664th part of the solar disc to be een, and this small portion afforded a light which appeared equally bright with Struts; whence he concluded the dis tance of Sirius to be 27664 times greater than that of the sun. Celsius appears to have been the first who proposed to mea sure light directly by means of what lie called a lueimeter. His method, how
ever, which was an extremely imperfect one, consisted simply in observing the greatest distance from the eye at which small circles painted on paper were dis tinctly visible in different lights. It was reserved for Bouguer to establish pho tomery on true principles. Having been induced by Mairan's remarks on the re lative proportion of the sun's light at the summer and winter solstice to investigate the experiment, he undertook a series of experiments: Lambert afterwards treated the subject more generally, and with great mathema tical elegance. The principle adopted by Bouguer and Lambert is extremely simple. Though the eye cannot judge of the pro portional force of different lights, it can distinguish in many cases with great pre cision when two similar surfaces presented together are equally illuminated, or when the shadows of an opaque object thrown upon them by different lights are equally dark. But, as the particles oflight proceed in straight lines, they must spread uni formly, and hence their density will di minis'h in the duplicate ratio of their distances. From the respective situa tions, therefore, of the centres of diverg ency when the contrasted surfaces be come equally bright, we may easily compute their relative degrees of illu mination. The objection to this method is, that the apparatus admits of no certain standard of comparison. Even the light of the sun itself, at the same altitude, and in the same climate, is subject to considerable variation ; much more so any artificial light, the force of which must always be influenced by a number of indefinable circumstances. In this re spect, therefore, the photometer describ ed in the preceding article has a great and decided. advantage: A simple and elegant application of the principle of Bouguer was made by the late Dr. Ritchie, of London. His appa ratus consists of f6 rectangular box, about an inch and a half or two inches square, open at both ends and blackened within, to absorb extraneous light. With in, inclined at angles of 45° to its axis, are placed two rectangular plates of plane looking-glass, cut from one and the same strip, to insure equality of their reflect ing powers, and fastened so as to meet at the top, in the middle of a narrow slit about an inch long , and an eighth of an inch broad, which is covered with a slip of fine tissue or oiled paper. In compar ing, by means of this instrument, the il luminating powers of two different sources of light, they must be placed at such a distance from each other, and from the instrument between them, that the light of every part of each shall fall on the reflector next it, and be reflected to the corresponding portion of the oil ed paper. The instrument is then moved nearer the one or the other, till the two portions of the paper corresponding to the respective mirrors are equally illu minated, of which the eye can judge with considerable certainty.
The modification of this method, which consists in contrasting the sha dows of an opaque object formed by dif ferent lights, is usually ascribed to Count Rumford, by whom it was pro posed, but was long before used by Lam bert. It is generally supposed that the equality of two shadows can he appreci ated with more certainty than that of two lights ; hut, when the lights are of differ ent colors, their estimation by either me thod admits of little precision.
M. Arago has proposed a method of determining the relative intensities of different lights entirely different in prin ciple from any of the preceding, and pro bably susceptible of much greater accu racy. It is founded on the properties of polarized light. When two lights are to be compared, the rays from each are polarized by causing them to pass through a plate of tourmaline cut parallel to the axis, or by reflecting them from a plate of glass, on which they fall at the polar izing angle. They are then received on a plate of rock-crystal, cut perpendicu larly to the axis, and observed through a doubly refracting prism. Each light will thus give two images tinged with the complementary colors. The images are i then brought into such a position that the red of the one falls over the green of the other. If the two lights are equal in intensity, this superposition will produce i a white image; if unequal, the image will be slightly colored with red or green, according as the one or the other pre dominates. The apparatus which this method requires is somewhat complicat ed, and its manipulation must be attend ed with considerable trouble.