On Negative After-Images, and Their Relation to Certain other Visual Phenomena . ry similar effects are observed when the disc described in Section Iis turned in the reverse direction. They naturally suggest that whitelight excites a blue or blue-violet sensation, the persistence of whichexceeds that of any other fundamental sensation. VII. External and Border Phenomena. Some very remarkable and interesting phenomena are exhibited inthe region of the visual field immediately adjacent to that upon whicha u pulsative after-image is being produced. It is a matter for sur-prise that one should be
On Negative After-Images, and Their Relation to Certain other Visual Phenomena . ry similar effects are observed when the disc described in Section Iis turned in the reverse direction. They naturally suggest that whitelight excites a blue or blue-violet sensation, the persistence of whichexceeds that of any other fundamental sensation. VII. External and Border Phenomena. Some very remarkable and interesting phenomena are exhibited inthe region of the visual field immediately adjacent to that upon whicha u pulsative after-image is being produced. It is a matter for sur-prise that one should be able to perceive after-images without detecting their Relation to certain other Visual Phenomena. 277 any indication whatever of the colours to which they are due, but it isperhaps even more surprising to find that parts of the retina uponwhich the intermittent white light does not fall may also be absolutelyblind to the exciting colour. The effect in question is conveniently demonstrated by the arrange-ment illustrated in fig. 9. A piece of clear glass, upon which is Fig. gummed a small circle of black paper or tinfoil, is fixed behind theiris-diaphragm P, fig. 3, and thus a round black spot, 0*6 cm. indiameter, is formed at the centre of the white-light disc projectedupon the screen L. In fig. 9 the outer circle represents the white-light disc, the shaded circle the colour-patch, and the inner one theblack spot upon the white-light disc. Suppose the colour-patch to begreen. When the apparatus is worked, the shaded circle becomespurple ; the site of the black spot, being illuminated five or six timesin a second by green light, might be expected to appear green; but ifviewed from a distance of 30 cm. or more it remains perfectly blackthroughout; under normal conditions no trace of a flicker of greenlight can be seen upon it. The apparent width of the blind regionadjoining the site of the pulsative image, therefore, exceeds half adegree. This induced blindness is most conspi
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