PHOTOMETER

(Gr. ^*f, light, and fjur^o,, measure.) An instrument for measuring the intensity of light, or of illumination. The most elegant instrument which goes under this name is the photometer invented 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 by a case of thin transparent gla , to defend 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 case; 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 gla sphere. The latter form of the instrument, though le commodious, is better adapted for nice observations; since, besides receiving the light more regularly, its balls, from being on the same level, are not liable to be any how disturbed in their indications by different strata of unequally heated air. The theory of this photometer depends on the a umed principle that the intensity of light is proportional to the heat excited by its incidence on the black ball. When the instrument is exposed to light, the rays which fall on the clear ball pa through it, without suffering obstruction; but those which strike the dark ball are stopt and absorbed at its surface, where, a uming a latent form, they act as heat, which, by expanding 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 counteracted by an opposite dispersion, caused by the rise of temperature which the ball has acquired. But, in still air, the rate of cooling is, within moderate limits, proportional to the exce of the temperature of a given surface above that of the surrounding medium. Hence the space through which the coloured liquid sinks in tiie stem will measure the momentary impre ions of light, or its actual intensity. {Leslie on the Relations of Air to Heat and Moisture.) The graduation is entirely arbitrary, and may be regulated according to fancy or convenience. Leslie adopted the same scale of divisions as in the differential thermometer, ten degrees of which correspond to one of the centigrade thermometer. 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 coloured ball stands at zero. In this country the direct impre ion of the sun at noon, about the summer solstice, forces the liquid down to 90° or 100°. The greatest force of the solar beams, in the depth of winter, measures only about 25°. At the altitude of 3° above the horizon, the whole effect of the sun's rays does not exceed one degree. The indirect light from the sky at noon in 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 found 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 le than half an inch in diameter, were transmitted to our planet, it would throw forth a light equal to the effect of 12,000 candles. It has been objected that the instrument now described is only a species of thermometer, and not strictly a photometer; since it measures heat, and not light. To this objection Sir J. Leslie replies by asking, " What does the thermometer itself indicate except expansion? As heat is measured by the expansion it occasions, so light is determined by the intensity of the heat which in every supposition invariably accompanies it. What other mode, after all, could be imagined for detecting the presence of light? How can an unknown quantity be expounded but in terms of one already known? " {Ency. Brit., art. " Climate.") It must be admitted, however, 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 splendour, and yet gives out far le heat than the comparatively dull combustion of hydrogen in the same gas; and the photometer is more affected by a fire so dull that not a single letter could be discerned in a well-printed page, than by the degree of daylight by which the same page could be read with pleasure and facility. For a particular description of the photometer, and an account of its numerous applications, see Leslie's Experimental Inquiry into the Nature and Propagation of Heat, 1804. [s. 936]