SUCCESSIVE INDUCTION OR AFTER-IMAGES 107 



Further, if Li at one retinal point gives rise to the same sensation as 

 Lo at another, previously stimulated by a different light, and M\ at 

 the first point gives rise to the same sensation as Mi at the second, 

 then L] + M\ will have the same action at the first as Li + Mo at the 

 second. As a special example, 'projjortiofial alteration of intensity of 

 two stimuli acting on retinal areas which have been previously exposed 

 to dift'erent excitations will produce the same alteration in sensation. 

 This is the " Law of Coefficients " (v. Kries^), adumbrated by Wirth^ 

 as the Fechner-Helmholtz law. From the law it follows that if the 

 change from normal produced by stimulation with three different 

 lights or light mixtures is known the change produced in any other 

 light mixture can be deduced. 



The law applies only to the photopic condition. It is therefore 

 least subject to deviation for foveal values. For peripheral values, 

 which are so subject to scotopic variations, and for stimuli of low in- 

 tensity, which so readily induce the scotopic condition, it ceases to be 

 valid. Moreover, with feeble stimuli the endogenous stimulus of the 

 intrinsic light becomes measurable relatively to the exogenous stimuli. 



When the primary and secondary lights are identical the result is a 

 gradual diminution of luminosity (local adaptation, Hering). Such a 

 diminution in differences of brightness occurs when two similar coloured 

 fields of unequal luminosity are fixed for a considerable time. Besides 

 the alteration in brightness there is also a diminution in saturation 

 with coloured lights, and further, though it is scarcely noticeable with- 

 out a comparison light, there is also a change in hue, Voeste^ found 

 that a yellow of wave-length 560 //./i, a green of 500 /x/x, and a blue of 

 460yLiyLt,show no appreciable change of hue. Hues between 500 and 560yu./Lt 

 change towards 560 yti/x, those between 500 /a^u, and 460 ^jx towards 

 460 /x/i. on prolonged fixation. These facts are of importance in the 

 equality of brightness method of estimating luminosity matches. In 

 Abney's* method " the angles of the sectors are rapidly altered from 

 " too light " to " too dark " and back again, and the range of angle 

 is gradually diminished until the observer sees both to be equally 

 bright." The change in brightness on prolonged fixation constitutes 

 one of the difficulties of this method with unintelligent examinees, 

 and is an argument in favour of the flicker method. 



If the primary stimulvs is white and the secondary stimulus coloured 



1 Nagel, p. 211. 2 Wundt's PMlos. Stud. xvi. 4 ; xvn. 3 ; xvni. 4. 



^ Ztsch.f. Psychol, u. Physiol, d. Sinne/forg. xviii. 257, 1898. 

 * Abney, p. 88. 



