PHOTOMETER (literally "light-measurer," from Or and senor), the name given to instruments constructed for the purpose of measuring the relative illuminating powers of different sources of light. When light or heat falls upon any substance, it is disposed of either by, reflection, absorption, or tranamiesion, or else by two of them, or all three of them combined. if two substances could be found which would reflect, absorb, and transmit calorific rays with the same inten sity, and likewise reflect luminous rays equally, but differ in their powers of absorbing and transmitting light, we should then possess the means of at least ascertaining whether the absorption of light alone will produce effects analogous to what is observed to follow the absorp tion of heat. For this purpose it would be only necessary to prepare • differential thermometer whose bulbs were of the substances possess ing the properties alluded to. The calorific rays accompanying the incident light would, by acting equally upon the two bulbs, produce no change in the indications of the instrument, and the only alteration, if any, which could ensue, would arise from the unequal absorption of light by the two bulbs. This alteration, however, when observed, though it might be considered a correct measure of the quantity absorbed, could not be taken for a measure of the quantity or brightness of the incident light, unless it could be further shown that the quantity absorbed by the same substance is proportional to the quantity of incident light, whatever may be its nature, that is, whether it be solar light, gas light, ite.
The photometer invented by Leslie differs from the instrument wo have supposed, merely in its being in some respects less deserving of the name. It consists of a differential thermometer having one of its bulbs of plain transparent glass, the other of the same material coated either with Indian-ink or black enamel. Leslie remarks :—" The rays which fall on the clear ball pass through it without suffering obstruc tion ; but those which strike the dark ball are stopped and absorbed at its surface, where, Resuming a latent form, they act as heat. This heat will continue to accumulate till its further increase comes to be counterbalanced by an opposite dispersion, caused by the rise of temperature which the ball has come to acquire. At the point of equilibrium therefore the constant accessions of heat derived from the action of the incident light are exactly equalled by the corresponding portions of it again abstracted in the subsequent process of cooling.
But in still air the rate of cooling is, within moderate propor tional to the excess of the temperature of the heated surface above that of the surrounding medium. Hence the space through which the coloured liquid sinks in the stem will measure the momentary impres sions of light, or its actual intensity." Allowing that the light incident upon the clear ball is wholly transmitted, and that that which strikes the dark ball is wholly absorbed, assumes a latent form, and then acts as heat, it by no means follows that thet effect produced upon the instrument was wholly or even chiefly attributable to the absorption of light, since we learn from Leslie's own experiments that the calonfie rays which accompany the incident light would be more abundantly absorbed by the dark than by the light ball. This has
since been so satisfactorily established by the observations of Thornam and others, that, as a measurer of light, the instrument may be regarded as useless.
The defects of Leslie's photometer were to a considerable extent obviated by the late Professor Ritchie, who, in 1825, communicated to the Royal Society the description of a new photometer. In order to intercept the caloriflo rays accompanying the light experimented upon, he transmitted the latter through a thick circular disc of glass into a metallic sir-tight cylinder, the diameter of which was considerable compared with its depth. The axis of the cylinder was placed horizon tally, and the aperture covered by the glass was the only one through which the light was admitted. Across the interior of the cylinder was stretched a circular sheet of dark paper, which absorbed the trans mitted light, and, as was supposed, thereby converted it into beat, which became sensible by its expanding the air within the cylinder. A second cylinder of the same form and construction was placed by the side of the first so that the line of axe. might coincide, but with the aperture for the admission of light turned in the contrary direction, and in that position they were connected by a bent thermometer tube containing • coloured fluid, which served to prevent the air of one cylinder from mixing with that of the other. So long as the air in the two cylinders poasessed the name degree of elasticity, the level of the fluid in the two branches of the tube was of course the same ; and a variation of level indicated a variation in the elasticity of the two bulks of air, arising from the more energetio action of the medium admitted through one aperture than through the other. To compare the relative intensities of two lights, the instrument was placed anywhere between them, and approached towards one or the other, until it was found that the position of the fluid in the tube was the same as when the instru ment was not under the influence of the lights. Supposing the whole of the calorific rays and none of the luminous rays to hare been inter cepted by the glass, this position determined the point at which the intensity of the two lights was the same ; and hence,since the intensity of light varies inversely as the square of the distance from its source [Lunn], it followed that at equal distances from their respective sources their intensities were directly proportional to the squares of their observed distances from the instrument.