D URA TION OF STIMUL US AND SENS A TION. 



1075 



giving a red and blue figure. These appearances required intense light, 

 and Burch l has recently described the spectrum as showing the three 

 colours, red, green, and blue, most strongly with intermittent light of 

 great intensity. 



Another phenomenon which has lately attracted much attention is 

 the appearance of colours, when a disc of black and white sectors is 

 rotated. These colours were first observed byFechner 2 in 1838, and 

 are often called by his name. He used a disc, shown in Fig. 389, 3 and 

 found that different individuals varied greatly in the distinctness with 

 which they saw the colours. The most usual appearance which accom- 

 panies flicker at a certain stage is one of a more or less definite pattern 

 in yellow and blue. The colours distinctly increase in vividness with 

 prolonged observation and with the brightness of the illumination. In 

 recent modifications of this experiment the appearance of colour has 

 been found to be more definite, if narrow black bands are placed on the 

 white sectors, or, better still, 

 if such bands are observed 

 through a gap in the disc. 

 All gradations of colour 

 may be observed depend- 

 ing on the relative position 

 of the black sectors, the 

 white sectors, and the bands. 

 The most definite colour 

 which is seen is red, and 

 Bidwell 4 has shown that 

 this is the same pheno- 

 menon as one described by 

 him, in which a bright 

 object on a dark background 

 appears, when suddenly ex- 

 posed, to be surrounded by 

 a narrow red border lasting 



about T V of a second. With 



FIG. 389. 



brighter illumination a dis- 

 tinct blue-green appears, 



and Bidwell 5 has referred this to the after-image of the red effect. This 

 appears to be an instance of the phenomenon already mentioned, in which 

 the after-image effect may neutralise or overpower the colour of the 

 original excitation to which it owes its origin. Some of these subjective 

 colours are said to be visible with monochromatic illumination. 



Fechner explained the phenomenon by supposing that the different 

 colours made their appearance at different rates, in the same way that 

 the coloured waning of after-images showed that they disappeared at 

 certain rates. This explanation was adopted by Helmholtz, 6 who sup- 

 posed that the process in the case of red and violet was more rapid 

 than for green, a supposition confirmed later by Kunkel (see p. 1068). 



1 Journ. PhysioL, Cambridge and London, 1897, vol. xxi. p. 431. 



2 Ann. d. Phys. u. Chem., Leipzig, 1838, Bd. xlv. S. 227. 



3 Fechner calls attention to the fact that, when this figure is revolved slowly in the 

 direction of the arrow, the black seems to extend beyond its limits ; when in the opposite 

 direction, the black seems to shrink, the latter effect being the more marked. 



4 Proc. Ray. Soc. L&ndon, 1896, vol. Ix. p. 368. 



5 Ibid., 1897, vol. Ixi. p. 268. ti "Handbuch d. phys. Optik," 1867, S. 380. 



