COLORIMETER, an instrument for measuring the depth of color in a liquid by comparison with a standard liquid. They are of three general types, based upon the three variables in the intensity.of colored solutions: (I) the quantity of colonng matter required to make the match; (2) the volume of the solvent in which it is held; (3) the thickness of the solution through which the light passes before it reaches the observer's eye. These principles are applied (1) when two solutions are brought to agreement in color by the addition of color ing matter; (2) when the match is effected by dilution of one of the two solutions; (3) when the solutions are made to match by changing the thicluiess through which the light beyond is forced to pass. The form most commonly in use is of the second type. It consists of two long, narrow glass tubes placed side by side on a stand. The standard liquid and the one in question are poured, in equal quantities, one into each tube; and water is then added to the darker till its tint becomes the same as the other. The amount of water used is a measure of the depth of color in the given liquid. The White colorimeter is the principal exponent of the third class. It consists of two hollow wedges of glass placed side by side, and ar ranged so that one may slide up and down on a bar upon which is marked a scale. The solution to be measured is placed in one of the wedges and the standard solution in the other. Both are viewed through a horizontal
slit. The wedge containing the standard is raised or lowered until the color is matched, and the scale is read.
Another type of colorimeter is employed to measure or test the colored light reflected from opaque bodies. The Ives colorimeter is of this type. It consists of a hollow box about 3 by 4 inches and 20 inches long, with a viewing lens at one end, a diffraction grating, and at the farther end lenses and slits which can be ad justed. The field of view is divided by means of a diaphragm so that the light to be tested appears in the upper half while the lower half is lighted by a mixture of red, green and blue violet rays from the grating. The slits through which the color rays come are opened or closed as the case may require, and the analysis is read on the three color scales. A new (1912) pre cision colorimeter constructed for the United States Bureau of Standards measures the wave length of the color which is to be tested. The instrument is so arranged that colored rays taken from a spectrum are mixed with white light to match the unknown hue; or, in the case of purples, the rays from the spectrum are mixed with the unknown to make white light. The instrument is operated through the use of a Lummer-Brodhum photometer cube.