376 PHYSIOLOGY CHAP. 



"When thin coloured sectors, interrupted in the middle by a 

 band which is half black and half white (Fig. 180), are rotated 

 on a white disc, the band should appear as a grey ring on a 

 slightly tinted whitish ground. Owing to contrast, however, the 

 ring is not seen as grey, but in the colour complementary to that 

 of the coloured sectors. 



This experiment, devised by Helniholtz, is only a proof in 

 another form of H. Meyer's earlier experiment, which shows that 

 simultaneous contrasts become plainer with pale colours in the 

 presence of light grey. Instead of covering the grey square on 

 the coloured background with semi-transparent tissue-paper, to 

 dilute it with white, Helmholtz obtained the same effect by the 

 physiological mixture of white and coloured and white and black 

 segments, that results on rapidly rotating the disc. 



From these examples it is plain that owing to simultaneous 



FIG. 179. Masson's disc for experiments on FIG. ISO. Helmholtz' disc for experiments on 



colour-contrast. colour-contrast. 



contrast a bright object placed beside a darker object becomes 

 brighter and more luminous, and vice versa ; and that a coloured 

 object in the vicinity of another that is not coloured (white or 

 grey) diffuses its complementary hue to the latter. 



Helmholtz interpreted the phenomena of simultaneous contrast 

 as errors of judgment. In the case, e.g., of coloured shadows we 

 mistake for white the yellow of the area illuminated by the 

 candle, and consequently assume that the shadows which are 

 really grey are bluish ; Hering, on the other hand, showed by a 

 variety of experiments that simultaneous contrast is not an error 

 of judgment, but that it depends on the spread of excitation to 

 the parts of the retina adjacent to those stimulated. 



XI. Since the time of Aristotle a number of hypotheses have 

 been put forward to explain the qualitative differences of visual 

 sensation, that is, of the perception of colours. The more gener- 

 ally accepted theories of colour- vision are undoubtedly that of 



