An instrument designed to exhibit the different quantities of light, especially in bodies illuminated in different degrees. In Lesslie's photometer, the essential part is a glass tube, like a reversed syphon, whose two branches should be equal in height, and terminated by balls of equal diameter: one of the balls is of black enamel, and the other of common glass, into which is put some sulphuric acid, tinged with carmine. The motion of the liquid is measured by means of a graduated scale; the zero is situated towards the top of the branch that is terminated by the enamelled ball. The use of this instrument is founded upon the principle, that when the light is absorbed by a body, it produces a heat proportional to the quantity of absorption. When the instrument is exposed to the solar rays, those rays that are absorbed by the dark colour heat the interior air, which causes the liquor to descend at first with rapidity in the corresponding branch. But as a part of the liquor which had introduced itself by means of the absorption is dissipated by radiation, and as the difference between the quantity of the heat lost and that of the heat acquired goes on diminishing, there will be a point where (these two points having become equal) the instrument will be stationary, and the intensity of the incident light is then estimated by the number of degrees which the liquor has run over.
Mr. Ritchie, of Nain, has constructed a very simple photometer, on the principle of Bougier. It consists of a rectangular box, about an inch and a half or two inches square, open at both ends, and blackened within for the purpose of absorbing irregular light. Two rectangular pieces of plain mirror are placed within the box, at right angles with each other, and at an angle of 45° with the sides of the box. A rectangular opening is cut in the upper side or lid of the box, about an inch long and an eighth broad, and, passing over the line formed by the intersection of the two mirrors, is half over the one and half over the other; the aperture is to be covered with a slip of fine tissue, or oiled paper. When used, it is to be placed in the same straight line, between the two flames to be compared, they being distant six or eight feet from each other, and is to be moved until the disc of paper is equally illuminated by the two flames. The illuminating powers of the two flames will then be directly as the squares of their distances from the middle of the photometer. In viewing the illuminated disc, it is well to look at it through a prismatic box, about eight inches long blackened within, to absorb strong light.
Sometimes, instead of using mirrors and the paper-screen, the Inclined plane9 are covered with white paper, and looked at directly through the aperture. However the instrument be used, a mean of several observations should be taken, the instrument being turned round each time. When the lights are of different colours, the plan Mr. Ritchie recommends is, to cover the rectangular opening in the instrument with a piece of fine white paper, printed distinctly with a small type; the paper is to be brushed over with oil, and then the instrument being placed between the lights, they are to be moved till the printing can be read continuously along the paper with equal ease on the one side as on the other. In the second form, the printed paper is to be pasted on the mirrors, or the inclined surfaces against which they lie, and is then to be read through the opening. It is advantageous to enlarge the openings in these applications of printed paper.