(from φῶς, light, and μέτρον, a measure) is the name of an instrument for measuring the intensity of light, whether it proceeds from a luminous body, or is reflected or refracted at different angles from opaque or transparent bodies. In our article on OPTICS, we have referred to the photometrical researches of Bouguer and Lambert, whose methods of comparing different degrees of illumination we shall now explain.
In order to measure the light reflected from a mirror B, Bouguer placed the light P between two surfaces of paper D, E, equally white and parallel to each other. He then placed his eye at A, so that he could see at the same time the paper E directly, and the image of the paper D reflected from the mirror B. When the direct and reflected images were brought into contact, the light P was moved along the line ED, so as to throw more or less light upon E and D, till their degree of illumination appeared perfectly equal. The light of E seen directly from A is then to the light reflected from B as EP is to DP.
The method used by Lambert is shown in the annexed figure.
Upward place a white square ABCD draw a line ILK, uniformly black; place a plate of glass EFGH at right angles to the white surface, so that the angle HIK is a few degrees less than a right angle. When the white surface ABCD is equally illuminated, the observer at O views the part HI projected into LQ, and the anterior part KL, seen by reflection PL, and will be able to find the position of the eye at O when the lines LQ and LP are of equal paleness, he measures the angle of incidence. In one experiment he found this angle 75½ degrees; and supposing the glass to be perfectly polished, he concluded that at that angle the quantity of reflected light was exactly equal to the quantity of transmitted light.
The method of equal shadows given by Count Rumford is shown in fig. 3, where L, L are the two lights to be compared, S an opaque cylindrical rod, and CD a screen of white paper. The lights are then placed at such distances from S that the shadows A, B are equally dark. In this case the squares of the distances of the shadows from the lights, that is LB², LA², are a measure of the relative intensities of the light. The rays LB, LA must be equally inclined to the surface of the screen.
The ingenious photometer of the late Mr William Ritchie, founded upon Bouguer's method, is shown in fig. 4, where ABCD is the section of a rectangular box, CF, DF the plane mirrors cut from the same plate, EG a rectangular opening covered with oiled or fine paper, and F a small division of blackened card to prevent the lights mingling with each other. The lights to be compared are then placed in the same straight line parallel to AB or CD, and equidistant from them, at the distance of 6 feet from each other, and the box is moved along this line till EF, GF are equally bright. The illuminating powers of the lights will be directly as the squares of their distance from the centre of the photometer.
The photometer of Sir John Leslie, which was merely his differential thermometer with one of its balls blackened, exhibited the difference of temperature of the two balls, and was not a measurer of light. Mr Ritchie invented a photometer having some resemblance to it, but differing essentially in principle. It is shown in fig. 5, the lower part of which is a differential thermometer, surrounded by two air-tight cylinders of tin ABCD, EFGH, composed of two plates of fine and thick glass AB, PG, and two circular tin plates CD, EH, the dotted line representing circular pieces of black bibulous paper. The light of one flame enters the glass plate AB, and the other PG. The heat is assumed to be absorbed by the thick glass and converted into heat by the black part of which is shown on the differential thermometer. When each branch of the thermometer is at zero, showing that the light converted into heat is equal on the two cylinders, then the intensities of the lights are directly as the squares of their distances from the instrument.
The photometer used by Professor Potter for measuring the light reflected at different angles by plane metallic specula, will be found in Brewster's Journal of Science, October 1839, vol. iii., new series, p. 284.
A photometer in which a variable light is produced by rotation, has been long ago proposed by Sir David Brewster. An opaque or a semi-transparent arm, ground glass for example, having its breadth decreasing to a point as it recedes from the centre of rotation, is made to revolve in front of a uniformly illuminated white surface, so as to produce every degree of illumination from absolute darkness to the light of the surface. Any law of variation may be obtained by the form of the side of the revolving arm being either rectilineal, curvilinear, or in steps, and by the angular distance between their extreme points. All light less bright than the surface is assumed to be measured by comparison with the reduced light at a distance from the centre of rotation. The same principle may be applied by reducing to equality with the other the brightest of two unequal lights received on a white surface by looking through the slits of a revolving disc.
Photometers have been constructed by viewing through a binocular instrument differently illuminated surfaces, and reducing them to an equality by different circular apertures applied to each eye. A photometer of this kind has been described by M. Doppler, and another has been recently proposed by Mr Mungo Ponton. Many objections, both physiological and optical, may be urged against the accuracy of such instruments.
The transition of a polarized ray from evanescence to light of a given intensity, and the determination of the law of variation by Malus, must have suggested to every person familiar with that class of phenomena the construction of a photometer.
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1 Vol. xvi., p. 335. 2 Photometria, part ii., c. i., p. 156. 3 Phil. Trans., 1794, p. 67. 4 Edin. Trans., vol. x., p. 443. 5 Phil. Trans., 1823, p. 141. 6 Edin. Trans., 1833-4, vol. xxii., p. 213. 7 Moligny's Recueil d'Oeuvres, &c., partie iv., p. 1752. 8 See Optics, vol. xvi., part vii., § 1, pp. 633, 634.