TRANSACTIONS OF SECTION I. 825 



colours, and in the other (B) to accentuate it. On whirling the disc, it is found 

 that the r.ate of revolution required to fuse the component colours in ring-band A 

 is less than that required for ring-band B. In this way the heightened contrast 

 between the colours is found to take effect when all knowledge of the contrast 

 between their components and the background has been eliminated from conscious- 

 ness. Judgment is thereby eliminated from the effect, and the relation of judg- 

 ment to simultaneous contrast decided against the lielmholtz view and in favour 

 of the Hering view. 



A measurement of the degree of simultaneous contrast may be obtained from the 

 rate of rotation required for fusion. 



2. On a parti-coloured disc two concentric circular bands, each composed of the 

 same two component tints, are so inscribed that the darker component of one (A) is to a 

 certain extent deepened in tint by simultaneous contrast against the background. On 

 spinning the disc it is found that the ring-band (A) appears darker than its fellow 

 ring-band, although physically the intensity of the components are exactly equal in 

 the two. This visual darkening is apparent w^hen all knowledge of the existence 

 of simultaneous contrast has been dismissed by fusion of the components of the 

 background by rapid translation of the surface. 



3. On a disc half black half white let two short black arcs jut from the black 

 into the white half, and at the opposite radius two counterpart white arcs jut into 

 the black half. These pairs are so placed as to compensate one for the other ; 

 throughout the entire disc the angular quantities of black and white are equal. 

 On spinning the disc the rate of intermission sufficient to extinguish ' flickering ' in 

 the sensation obtained might be expected to be the same for all parts of the disc. 

 This is not the case. In one direction of spin, the rate of rotation required to fuse 

 the ring-bands possessing the jutting black arcs is higher than that required for 

 the intermediate ring-band on the disc; in the opposite direction the reverse. 

 Successive contrast is here adding its effect to simultaneous contrast : the latter is 

 here, as in the previous experiments, obviously taking effect, although rapid trans- 

 lation of the surfiice has removed all possibility of the observer being aware of its 

 existence on the disc. 



4. On a disc half white half black two short red arcs (A and B) are inscribed in 

 tlie black half at different radial distances, and two similar short arcs of black 

 (A' and B') are inscribed in the white half, A' and A, B and B,' being at the same 

 radial distances. On being whirled the tints of the two ring bands are found to differ 

 in brightness, even when in an ordinarily lighted room, the rate of intermission is 

 as rapid as 80 times a second. This difference seems explicable by successive 

 contrast, and indicates that even after a fiftieth of a second exposure to black, the 

 eye has been more sensitive to white, and conversely after a fiftieth of a second 

 exposure to moderate white. 



5. On a disc of 160° white and 200° black, let some short and rather narrow arcs 

 of red be placed on the white sector where it abuts on the black. Let half the 

 number of red arcs lie at one border of the white near the edge of the disc, the 

 other at the other border near the centre of the disc. When studied by lamp 

 light (j-ellowish illumination) one of the sets of arcs will, on spinning the disc 

 somewhat slowly, seem much less bright than the other set, and the grey of the 

 disc in the spaces between the arcs of visually darker red will appear bluish-green ; 

 in the spaces between the visually brighter red arcs will appear pale yellow. 

 This yellow appears due to a development of a positive after-image, the blue 

 chiefly to a negative image, but also in part probably to simultaneous contrast. 

 On whirling the disc at higher speed, the tints of the two red bands, also of the 

 intermediate bands, approximate, both the latter two becoming pale greenish-blue. 

 At still higher speeds the red bands become fully alike, and the intermediate bands 

 become completely similar pale green-blue bands. At this rate the intermittence 

 has become too frequent to permit the influence of rebound effects, and successive 

 contrast has been eliminated, simultaneous contrast alone remaining. But the rate 

 required to do this is higher in certain discs than one-fiftieth of a second. By 

 thus using the discs as rheotom for the visual sensations, it is found that a 

 perceptible after-image is formed after a very moderately intense stimula- 



