The same gentleman afterwards constructed a very simple instrument which affords a good measure of the relative brightness of two lights, provided they are of the same colour. The principle originated with Bouguer, who published it in his Trait6 d'Optique,' in 1760. The annexed figure represents a vertical section of the instrument. It consists of a rectangular box open at both ends and blackened upon its inner surface. On the top is a long narrow rectangular slip A /3, covered with tissue or oiled paper. Within are two sheets of plane looking-glass, o D and C E, cut from the same slip to ensure uniformity of reflection. Each sheet has the same width as the box, and its length equal to the hypothenuse of a right angled isoseelea triangle, whose side is the height of the box. Their reflecting surfaces are turned towards the open ends of the box, and their upper extremities rest against each other along a line, which in the figure is projected into the point c, and which divides the aperture A n into two equal parte, separated by a narrow strip of black card to prevent the mingling of the lights reflected from the two planes. In using the instrument it is placed between the lights whose intensities are to be compared, so that they may be reflected from c D and o z upon the tissue paper A B. his then approached nearer to one or the other until, to an eye situated above A B, the two portions A c and n o appear equally illuminated, which, on account of the immediate proximity of A o and n may be determined with tolerable correctness, the colour of the two lights being supposed the same. The distances of the lights from the vertical ce being measured and squared, give the direct ratio of the intensities.
There is a mode of comparing the illuminating powers of two lights suggested by Count Rumford, which is remarkable for the facility with which it may be applied, and the simplicity of the requisite apparatus, nothing more being needed than a smooth surface of small extent and of a light uniform colour, and a blackened stick for throwing a shadow. The surface is illuminated by the two lights experimented upon, which are to be so placed, that when the stick is interposed between them and the surface, the two shadows may be nearly in contact, which will enable the eye to decide whether they are of equal depth, and will at the same time ensure the intercepting of rays equally inclined to the surface. So long as the shadows are of unequal depth, one of the lights must be brought nearer to or withdrawn farther from the surface till an equality of depth is obtained, and then the squares of the perpendicular distances of the lights from the surface give the ratio of their intensities. If an equality between the inclination of
the intercepted rays to the surface cannot be obtained, then, when the two shadows are of the same depth, the intensities of the lights will be directly proportional to the squares of their perpendicular distances from the surface, and inversely proportional to the sins' of the inclinations of the intercepted rays to the surface.
Suppose, for example, it were required to ascertain the illuminating power of a gaslight burning 5 cubic feet of gas per hour, as compared with that of a sperm candle burning 132 grains of spermaceti per hour ; suppose the screen to be at 100 inches from the gas-light, and the candle only 2715 inches in order to equalise the shadows. The relative intensities of the two lights are then found by squaring the distances of each light from the screen ; the gas-light will diffuse a light which bears the same proportion to that of a candle as or as 16 to 1.
Wheatstone's photometer consists of a small bright bead, to which a rapid looped motion is imparted, and this being placed between the two lights, both are seen reflected from the different points of the bead's surface. By the principle of persistence of impressions on the retina the two looped curves are seen of different degrees of bright ness; the brighter light is to be removed until these curves appear of the same brightness, and the intensities of the lights are then as the squares of the distances as before. Bunsen's photometer consists of a compound disc of paper, of which the inner white disc is opaque, and the outer surrounding ring is made translucent by painting it with oil. When this is placed between the two lights, and viewed on one side, the opaque disc will be seen by reflection, and the translucent ring by transmission from the other light. The disc is then to be moved until both it and the ring appear to be equally illuminated, when the distances may be taken and squared as before. M. Babinet has con trived a polarising photometer. There are also photometers for measuring the relative brilliancy of the stars. These depend upon this principle :—that the brilliancy of the image of a star' as seen through a telescope depends, eceteris paribus, on the size of the object glass ; by obscuring the outer rings of the object-glass the image of a star of high magnitude can be reduced to the light of a star of lower magnitude, as seen through the entire object-glass. The relative brilliancy of the two bodies may then be inferred from the diameters of the free object-glasses employed. In practice it is better to make use of two corresponding telescopes simultaneously. [STARS.]