In constructing photometers, or instruments for measuring the comparative brightness of two or more lights, it must be borne in mind that the human eye cannot judge direotly, with any approach to accuracy, of the relative intensity of two lights, although it can determine readily whether two shadows are of equal intensity, or whether two similar surfaces of equal extent are equally brightly illuminated. Tho other principle involved in the use of photometers is the law that when two shadows of tho same object, produced by different lights, are equally intense, or when two similar surfaces are equally illuminated, the intensities of the two lights producing this effect vary directly as the squares of their distances from the screen. Thus, if a lamp at 2 ft. and a candle at 1 ft. from the acreen produced the same shadow or illuminated surface, aince the square of 2 is 4, aud the square of 1 is 1, the relative intensities of the two would be as 4 to 1, i. u. the lamp would bo of " 4-candle power." It is neoemary to fix upon some standard, in terms of which the brightness of any given light may be expressed. Various standards have been proposed at difforent times, such as lamps of a definite construction, burning oil of a fixed quality at a given rate per hour—for example, the French caroel lamp, or better, Parker'a hot-oil lamp, but it has been found that the most uniform or least variable standard of illumination, is a wax (or spermac,eti) candle, size 3 to the lb., with a wick of 27 or 28 threads of the best Turkey cotton, and burning at the rate of 125 gr. an hour. It is a candle of this kind that is referred to in speaking of gas as " 16-candle," or of an electric light as of 400-oaudle or 6000-candle power. The standard for gas-testing, as fixed by Act of Parliament, is a sperm candle burning at the rate of 120 gr. an hour, or 2 gr. a minute. In the best photometers, as will be seen presently, the standard light is fixed upon a balance, so that its rate of burning may be constantly checked.
Before proceeding to describe the various kinds of photometers, it may be useful to consider briefly two remarkable resulta of careful photometric observation, which have an important practical bearing upon the most economical arrangement of a number of separate lights, whether they be candles, lamps, or gas-flames, i. e. the arrangement which will give the greatest total amount of light from the various illuminating sources.
The first is that when the flames of two lamps or candles touch each other, the luminous intensity of the combined flame is greater than the SUM of the intensities of the separate flames. This effect was first observed by Dr. Benj. Franklin, and appears to be due to the increased temperature at the part where the flames overlap.
This fact has been taken advantage of by W. Sugg, in the construction of those combinations of 2, 3, or 4 flat-flame gaa-burners, now so much used in the standard lamps placed at the inter sections of streets, and other important points in many large English and Colonial towns. The gas argand burner is also an extreme in8tanee of the kind, the ring there being made up of a series of round-hole jets, eaoll [Angle flame of which overlaps the flame on either aide of it.
The second result may be thus expressed :—A comparison of the amounts of light afforded by the same number of flames in different relative positions proves that flame is perfectly transparent, and thus that the luminous effect of a row of lights is the same whether this arrangement is parallel with or perpendicular to the direction of the rays ; similarly a flat gaa-flame gives the same degrre of light in all directions.
The chief forms of photometer will now be deecribed. The simplest, most readily constructed, and most easily used, is that known as Rumford's. It conaists merely of a black cylindrical rod mounted vertically upon a stand or foot, and of a white screen upon which to receive the shadows of the rod. The lights to be compared (all others should be put out) are plrieed about until the respective shadows cast by the rod are nf equal depth. The distances of the lights from the screen are then carefully measured, and each number thus obtained is multiplied by itself. The propor tion between these products represents the relative intensities of the lights under examination. For example, suppose larnp A at 21 in. and lamp B at 30 in. from the screen gave equally deep shadows, then, since 21 x 21 = 441, and 30 x 30 = 900, lamp A is to lamp B as 441 to 900, or nearly as 1 to 2, or, in other words, lamp B gives twice as much light as lamp A. As a similar calculation has to be made in all photometric tests (though it is frequently assisted by previously constructed tables suited to each instrument), it will not be repeated.
Bunsen's photometer depends on the equal illumination of two surfaces, and is much more exact than the preceding. The principle of it, with very slight modifications, is adopted in the delicate photometers used in gas-testing. The essential part of it is a piece of thin paper stretched in a frame, and the paper is rendered semi-transparent by being saturated with a solution nf spermaceti in turpentine-oil, with the exception of a central spot about 0.75 in. in diameter, which is allowed to remain opaque. In using it (in a dark room), the standard light is placed behind the spot, and the variable one in front. When the two surfaces are equally illuminated, the opaque spot dis appears, and the whole surface of the disc is perfectly homogeneous in appearance.
Wheatstone's photometer consists of a small silvered polished bead, mounted upon a stem to which a looped motion is given by appropriate clockwork. When it is placed between two sources of light, and the clockwork is set in motion, two looped curves of different brightness are seen, so very close together that their intensities can readily be compared ; the lights are then adjusted to give curves of equal brightness, and their respective distances are read off. The formation of a luminous curve by a moving bright bead, depends, of course, upon the well-known principle of " persistence of vision," the simplest illustration of which is the circle of fire traced by a lighted stick whirled round by band.