2. Reduced by its own iron (if ferrous sulphate).
PHOTOM′ETRY. The art of determining the relative intensities of different lights. Various methods have been adopted, at different times, for this purpose, among which, however, a few only are sufficiently simple for general application. The principle adopted by Bouguer and Lambert depends on the fact that, though the eye cannot judge correctly of the proportional force of different lights, it can generally distinguish with great precision when two similar surfaces or objects presented together are equally illuminated, or when the shadows of an opaque object produced by different lights are equally dark. Now, as light travels in straight lines, and is equally diffused, it is evident that its intensity will progressively lessen as the distance of its source increases. This diminution is found to be in the duplicate ratio of the distance. To apply this principle to candles, lamps, gaslights, &c., we have only to arrange two of them so that the light or shadow resulting from both shall be of equal intensity, after which we must carefully measure the distance of each of them from the surface on which the light or shadow falls. The squares of these distances give their relative intensity. In general, some known light, as that from a wax candle (4 to the lb.), is taken as the standard of comparison.
Dr Ritchie’s ‘photometer’ consists of a rectangular box, about 2 inches square, open at both ends, and blackened inside to absorb extraneous light. In this, inclined at angles of 45° to its axis, are placed two precisely similar rectangular plates of plain silvered glass, and fastened so as to meet at the top, in the middle of a narrow slit about an inch long and the eighth of an inch broad, and which is covered with a strip of tissue or oiled paper. In employing this instrument, the “lights must be placed at such a distance from
each other, and from the instrument between them, that the light from each shall fall on the reflector next it, and be reflected to the corresponding portion of the oiled paper. The photometer is then to be moved nearer to the one or the other, until the two portions of the oiled paper corresponding to the two mirrors are equally illuminated, of which the eye can judge with considerable accuracy.”
In Prof Wheatstone’s ‘PHOTOMETER’ the relative intensity of the two lights is determined by the relative brightness of the opposite sides of a revolving silvered ball illuminated by them.
In the method of photometry usually, but erroneously, ascribed to Count Rumford, the shadows of an opaque object formed by different lights, and allowed to fall on a white wall or paper screen, are contrasted. A wire about 3⁄16ths of an inch thick, and about a foot in length, with the one end bent so as to form a handle, is commonly used to form the shadows. The method of proceeding is similar to that first above noticed.
It is generally supposed that the equality of two shadows can be appreciated with greater certainty than that of two lights.
PHTHIS′IC. A popular name for difficulty of breathing, from its supposed resemblance to phthisis. See Bronchitis, and below.
PHTHISIS. (A wasting away.) This is the formidable disease ordinarily or popularly known as “consumption,” although, strictly speaking, there are points of difference between consumption and phthisis, as well as between these and another variety of the malady known as tuberculosis. The statistics which follow, however, apply to all those diseases of the lungs accompanied by wasting, and as such include the mortality from phthisis, tuberculosis, and consumption.
In the Registrar-General’s returns for many years past, under the heads ‘phthisis,’ or ‘tubercular disease,’ have been included chronic bronchitis, emphysema, fibroid changes