102 COLOUR VISION 



conditions the reverse is the case. If the primary stimulus is an area 

 of white light which is sharply defined and surrounded by a dark back- 

 ground the negative after-image, bordered by a halo as described, 

 is observed. If however the transition from the illuminated area to 

 the background is made gradual a positive after-image is almost 

 invariably obtained. McDougall used a ground glass disc, 12 cm. in 

 diameter, illuminated from behind. On the far side about a dozen 

 sheets of white paper were pasted, each with a circular hole in it con- 

 centric with the edge of the disc. Of these holes the smallest was 

 2 cm. in diameter, and each of the others was about 1 cm. larger in 

 diameter than the preceding one. Such a " shaded disc," when 

 illuminated, showed a central evenly lighted circle, 2 cm. in diameter, 

 surrounded by a zone, 5 cm. in breadth, in which the brightness 

 diminished regularly to the periphery, where it became negligible. 

 The shaded disc gives with white light a positive after-image without 

 any halo. On the other hand with colours, though the after-image 

 is often homochromatic in the first one or two seconds, with most 

 intensities of light it is approximately complementary through almost 

 the whole of its course. In the case of red and green especially it is 

 very constantly the rule that red predominates in the after-image of 

 green, and vice versa. Sufficient attention does not appear to have been 

 paid hitherto to this fundamental difference between the relation of 

 black to white and that of green to red and blue to yellow in such after- 

 images from " shaded " lights. 



Though positive and negative after-images are opposed there 

 is no discontinuity between them. If the positive after-image is 

 developed and the eyes are then uncovered and directed towards a 

 uniform moderately illuminated field the negative after-image at once 

 appears. If the illumination of the field is suitably chosen no after- 

 image is seen. The nature of the after-image depends therefore upon 

 the nature and intensity of the primary stimulus and upon the nature 

 and intensity of the secondary stimulus. Great diversity of after-images 

 may therefore be obtained. The after-image of a white spot may be 

 coloured^ ; that of a coloured spot may not be accurately the comple- 

 mentary of the primary stimulus. Anomalous colouration may be due 

 to light passing through the sclerotic and the iris, or to abnormal 

 conditions of the retina (Hilbert^). Moreover, Burch^ has brought 



^ Aristotle; Goethe, Farbenhhre (1819), Eastlake's trans, p. IG, 1840. 



2 Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. iv. 74, 1893. 



3 Proc. Roy. Soc. Land. B. lxxvi. 212, 1905. 



