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 but the results which are obtained vary with the color excitation curves of the given observer.

A new and promising application of the trichromatic method of color analysis is to be found in Jones’ subtractive colorimeter (). Although the psychophysical principles involved in this instrument are substantially the same as in three-color analyzers based upon additive mixture, the physical operation of the instrument is quite different from that of a three-color additive color-matching device. White or tungsten light is passed in succession through three wedge filters which absorb respectively red, green and blue, and an adjustment of the three wedges can be found which yields a color-match with the sample which is under examination. A neutral wedge is also provided in the path of the beam, permitting a match to be obtained by the use of only two of the chromatic wedges in combination with the neutral one.

D. .—Several partial systems of color specification have been based upon the rotatory dispersion of quartz, the degree of this dispersion being a function of the thickness of the quartz plate which is employed ('; '). By placing such a plate properly between nicol prisms or other polarizing devices, variations in the relative angular positions of the latter with respect to their extinction positions may be employed to determine a wide variety of spectral transmissions, all of which, however, follow a definite law. Still greater flexibility is obtained by the use of two quartz plates and three nicol prisms. A radiation source of known energy distribution is ordinarily utilized. Instruments based upon this principle are the Arons chromoscope and the leucoscope, recently studied very thoroughly by Priest. The principle appears to have promise of very wide applicability, especially as a means of producing and of specifying a large variety of spectral distributions which it is difficult or impossible to obtain with filters or original sources of radiation. In general, however, the method depends upon the matching of apparent colors, rather than of identical energy distributions.

E. .—The series of spectral distributions determined by successive values of &Tau; in Planck’s