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Table 1

Equivalents in Terms of Fresnels of Wave-lengths in Millimicrons

Wave-Length Millimicrons		Frequency Vibrations/Seconds 1012	Wave-Length Millimicrons			Frequency Vibrations/Seconds 1012	Wave-Length Millimicrons		Frequency Vibrations/Seconds 1012
	400	750.0	He		492.2	609.5		600	500.0
Hg	404.7	741.3			500	600.0		610	491.8
Hg	407.8	735.7	He		501.6	598.1	Hg	615.2	487.7
	410	731.7			510	588.3		620	483.9
	420	714.3			520	576.9		630	476.2
	430	697.8			530	566.1		640	468.8
H	434.1	691.1			540	555.6		650	461.6
Hg	435.8	688.4	Hg		546.1	549.4	H	656.3	457.1
	440	681.9			550	545.4		660	454.6
He	447.2	670.8			560	535.8	He	667.8	449.3
	450	666.6			570	526.2		670	447.8
	460	652.2	Hg	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	576.9	520.0		680	441.2
	470	638.4			579.1	518.0		690	434.8
He	471.3	636.5			580	517.2		700	428.6
	480	625.0	He		587.6	510.5	He	706.5	424.6
H	486.2	617.1			590	508.5		710	422.6
	490	612.3
Hg	491.6	610.2

A. Psychophysical Functions.—There are two important, general types of psychophysical functions which occur in colorimetrics, (1) a type which expresses a direct relation of dependency between a psychological color attribute (vide supra)—measured in threshold steps—and a stimulus variable, and (2) a type which formulates relations between two or more stimulus variables, such relations depending upon and expressing the conditions for the equation in one or more psychological dimensions, of the colors due to different stimuli.^[1]

B. Color Excitations and Psychophysical Primaries.—An example of the second type of function appears in the color excitation curves for various visual color systems, which curves show in what proportions of intensity a number of selected color

1. A further type specifies the stimulus conditions for just-noticeable (or otherwise standardized) differences between the colors evoked by compared stimuli. On certain assumptions, functions of this type can be integrated to yield those of type (1).