Photometry (Gr. , light, and measure), the process of measuring the intensity of light. The first quantitative comparison of different sources of light with much approach to accuracy was made by Huygens about the middle of the 17th century. He used a tube having a small aperture at one end, in which was placed a minute globular lens which allowed the 27,664th part of the sun's disk to be seen. This fraction of his light being equal in brightness to the star Sirius, Huygens concluded that the distance of Sirius from the earth was 27,664 times as great as that of the sun. (Hugenii Cosmotheoros.) Bou-guer was led, by observations of Marain on the relative intensity of the sun's light at the summer and winter solstices, to make an extensive series of investigations on the subject, and his results were published in his Essai d'optique (1729). His photometer compared the reflecting powers of two different surfaces by having the image of one reflected in a mirror which was placed in a line with the other surface and the eye of the observer.

The two re-. flecting surfaces had a light placed between them, which was moved from one toward the other until the reflection of the one in the mirror was equal in intensity to the direct light from the other; and as that intensity is in proportion to the square of the distance, the reflecting or absorbing powers of the two surfaces could be readily computed by measuring the distance of the light from each of the surfaces. Lambert, ten years later, published his Photometria, a work of great mathematical elegance. He describes a photometer by which the reflecting and transmitting powers of different translucent materials may be accurately ascertained for various angles of incidence. The photometer of Bouguer was modified by Ritchie so that a comparison of sources of light could be made, as well as the reflective powers of surfaces. A box with the axis placed horizontally has two plane reflectors, c b and e d, cut from the same plate, placed with two edges at right angles at c so that rays of light parallel with the axis of the box will be reflected perpendicularly upon the oiled paper screen at a, a blackened diaphragm being placed at a c.

When the lights are placed so that the illumination on both sides of a is the same, the square of the distance of the lights from the mirrors will give the comparative intensity. Rumford's photometer, valuable because it may be easily extemporized, employs the principle of comparing the depth of shadows, and consists of a vertical staff placed a short distance in front of a screen of tissue or oiled paper. The shadows may be compared in front, or on the back side of the paper, the latter method being preferable because the back of the paper may be in a darkened chamber, thus allowing the eye to be undisturbed by the glare of the lights. Bunsen's photometer consists of a screen of thin writing paper stretched on a frame and marked with a grease spot. If a light is placed on each side of the paper, the spot viewed from the side of strongest light will appear darker than the surrounding space; from the other side, lighter. When the light falling on each side of the paper is equal, the spot and the surrounding surface will present the same shade, and the squares of the distances of the sources of light will respectively indicate their intensity. A modification of this instrument by Dr. Letheby is in common use in gas works.

Masson's electro-photometer has the advantage of comparing lights of different colors. A circular disk, divided into equal white and black sectors, makes by clockwork from 250 to 300 revolutions a second. When illuminated by a constant source of light, the disk has a uniform gray tint, because of the blending produced by the duration of the visual impression; but when lighted instantaneously by the electric spark, the black and white sectors appear distinct and as if fixed. If now the constant light be continued, the intensities of two electric lights, of the same or of different colors, may be compared by removing one and then the other from the disk, until the latter appears uniformly gray. Conversely, two constant lights may be compared by carrying each toward the disk till it is uniformly gray, the illumination being primarily made by one source of electric sparks. The square of the distances in either case will be proportional to the intensities of the lights which are the subjects of examination.

The general conclusions obtained from photometric observations are that flame is perfectly transparent to light; so that the luminous effect of a series of flames placed side by side is the same whether the light is received in a direction perpendicular or parallel to the line in which they are placed. In like manner a flat gas flame, such as that of a bat's-wing burner, casts the same amount of light in all directions. It has been found that when two lights are compared, of such relative intensity as that one should cast a shadow on a screen 64 times as strong as that of the other, the lesser light will in reality cast no shadow at all. Therefore it is concluded that light emanating from any source becomes imperceptible in the presence of light having 64 times its intensity. Hence, whenever the sun's light in the morning becomes 64 times as strong as that of a star, the latter will disappear; and in the evening, when it becomes less than 64 times that of a star, the latter begins to be visible. When two flames overlap each other, the intensity of the combined flame is greater than the sum of the two separate flames. This effect, first observed by Franklin, appears to be due to the increased temperature of the part where the flames overlap.

Arago and Fres-nel constructed lamps for lighthouses on this principle, in which several concentric wicks were arranged near to each other. - Chemical Photometers. At the end of the article on Light mention is made of investigations in regard to the measurement of the chemical action of light by Dr. John W. Draper, in which he obtained his results by determining the amount of hydrochloric acid formed in a given time from the union of its gaseous elements under the influence of light. Bun-sen and Roscoe have since extended these experiments, and have produced an instrument on the same principle which is capable of the most delicate measurements. It consists of three parts: 1, an apparatus for generating the two gases by electrolysis of hydrochloric acid; 2, one for exposing the gases to light; and 3, one for measuring the amount of hydrochloric acid generated by recombinations of the hydrogen and chlorine. The gas-generating apparatus consists of a battery of three or four cells connected with two carbon electrodes immersed in a vessel containing hydrochloric acid. The gases are passed through waste bulbs containing water into an insolation vessel, which has its lower part blackened, receiving the light upon its upper surface, by which the gases are made to combine.

Beyond the insolation vessel is a measuring tube, beyond this a vessel of water, and after this one containing slaked lime and charcoal, to absorb chlorine. The gases are first passed through the whole apparatus until the water is saturated, when the insolation vessel is exposed to the light and the connection between it and the generating apparatus closed. Combination of the hydrogen and chlorine gases then takes place with diminution of volume, which causes the water to return through the measuring tube. The rate of movement of the water shows the diminution of volume, and consequently the amount of chemical union. Their unit of light was obtained by employing a flame of pure carbonic oxide gas burning from a platinum jet of a certain diameter and issuing at a certain rate. The apparatus is useful in measuring the chemical action of diffuse daylight, of direct sunlight, of flames from different sources, and of the different parts of the solar spectrum. A chemical photometer for registering meteorological changes was also devised by Bunsen and Roscoe, and called a pendulum photometer, by which sensitive paper was exposed to the action of light for a definite time by means of the oscillation of a pendulum.

This instrument was used to measure the varying intensity of the sun's light under different positions of the sun, and of conditions of the atmosphere. Descriptions of these instruments and of the experiments made with them may be found in the "Philosophical Transactions " of the royal society of London for 1857, '59, '62, and '63.

Bouguer and Ritchie's Photometer.