Page:Philosophical Review Volume 2.djvu/373

No. 3.] mentary-sensation curves, is not a certain criterion of the two types, because of the individually different absorption of light in the pigment of the macula lutea.

(4) TRICHROMATIC SYSTEMS. – § 11. General properties of such systems. Nearly all women, and 96 p. c. of men are trichromates. Common to all these systems is the possibility of expression in terms of Newton's color-table. The construction of this latter makes the assumption of at least three elementary sensations necessary. But the relations of saturation show that there are at most two such represented in the spectrum; the triangle must be ideally supplemented. The "terminal distances" are present here, as for dichromatic systems. Inwards from these come the intermediate distances, – as it were, dichromatic spaces. Centrally lies the trichromatic middle distance. No shade of the intermediate distances can be produced by mixture of the light of the terminal distances; the former point to a new elementary sensation, and to the same: since else there would be four elementary sensations, – which is contradicted by experience. – Trichromatic systems are normal and anomalous. § 12. Complementary colors. These are either homogeneous or mixed (with white): the former use of the term is that of this paper. They were determined for the gas-light employed in the investigation. The form of the curve agrees with that of Helmholtz, and of von Kries and von Frey. § 13. The two groups of trichromatic systems. (Cf. § 11.) The test is the mixture of (lithium) red and (thallium) green to equal (natrium) yellow (Rayleigh, Donders). Mixtures which have reference only to the blue and violet parts of the spectrum are valid for both forms of the system. (a) Normal trichromatic systems. § 14. The choice of color equations and the direct results of the observations. The color equations were obtained in the face of two difficulties. For accuracy of judgment whitish colors must be avoided, and therefore parts of the spectrum mixed, which lay tolerably near one another. For accuracy of calculation, the parts must lie as far from one another as possible. Three forms of equation were obtained: for the terminal distances; where a mixture of two components gave the intermediate color without noticeable diminution of saturation; where there were two components on either side of the equation. §§ 15, 16. Calculation of the elementary-sensation curves R, G, and V. § 17. Survey of results: testing of the elementary-sensation curves by complementary colors. The ordinates of the curves refer to the dispersion gas-light spectrum; they are transformed into diffraction