while it invalidates the distinction of ground-sensations. The apparently exceptional position of purple in the color-series is due solely to the arbitrary conditions of 'normal' dispersion. Since all component theories are merely mathematical schemata, the Helmholtz classification of color-vision under the heads of dichromatic and trichromatic must be given up: seeing persons are rather achromates, dichromates, or polychromates. (2) Diagnosis of color-blindness. Normal color-vision presents differences in different persons. The extent of the spectral colors is variously defined: the spectrum is of various length; the brightness-maximum is variously placed; there are differences between the two eyes. Color-blindness implies considerable deviation from the norm. To test it one may use (a) the pseudoisochromatic tables of Stilling (not certain); (b) the spectrum; (c) colored wools; (d) colored papers, employed directly or in top-mixtures; (e) colored flames; (f) colored glasses and combinations of gelatine sheets. It is best to submit the subject to all these tests. (3) Experimental results. (a) A bipolar system, with saturation-maxima in the position of (the normal) yellow and indigo-blue. Neutral line near E. (b) The color-series is interrupted at three points; pure yellow, blue-green, and purple give only the brightness-sensation. Brightness-maximum in the normal place. (c) Red-green blindness; the neutral line at λ=518. (d) A blue-yellow system, with unshortened spectrum, (e) Hereditary and monocular partial color-blindness. The color-series lacks the qualities violet, green, and yellow, and the transitions of these to neighboring qualities. A unique case. The color-blind eye sees everything brighter than the normal eye. This the author interprets to mean that in all instances, where no energy is expended in the excitation of a color-sensation, the whole stimulation- energy can be applied to the production of the brightness-sensation. The color-sensibility is apparently greater in indirect vision. (f) A case only different in degree from that just mentioned. (Subject brother of the last.) (g) A system which proves that the absence of a quality (in direct perception or as contrast) does not necessarily involve the absence of the opposite quality.
E. B. T.
(1) The psychological basis of the comparison of small time-magnitudes. 1. No adequate theory has as yet been propounded. The conscious content in question consists of the secondary impressions of expectation-strain and surprise. This is indicated both by the results of experiments with variously graduated series of metronome-beats and by contrast--
