458 Proceedings of Royal Society of Edinburgh, [july 16 , 
certain. But contrast alone cannot explain all. It is hard to see 
why, in the first stage of this experiment, green should he the con- 
trast colour to yellow. Let us say that the red fibres are enormously 
stimulated in the part of the retina covered by the image, hut so are 
the green, since yellow is the resultant. 
In the second stage a reddish hand appears outside the green 
edges. This might he plausibly explained as a contrast efiect. It 
might he said that corresponding to the position of the green the 
red fibres are completely exhausted, while farther out they are not. 
But in the next stage, when the image is altogether red, there is no 
contrast effect at the edges, and it was always seen that the redden- 
ing of the central part occurred with a somewhat higher speed than 
the reddening of the edges. It would seem that the latter is, so to 
say, a forerunner of the former change, and the probability there- 
fore is that it is due to the same cause. 
Separate experiments have shown, as already stated, that where 
the light is less intense the phase of the appearances is more 
advanced, with any given speed, than where it is more intense. 
Where, for example, two separate lights of unequal brightness 
are so adjusted that the two images fall near one another, the 
weak light behaves itself like the edge of the bright one. The 
edge of the image in all our experiments was manifestly less 
bright than the middle part. With the small mirror the images 
reflected from the surfaces of the glass intensified the difference, as 
they were much less brilliant than the image given by the silver. 
In Experiment 1, it is well seen how the appearances at the edges 
are related to those in the central part. With the mirror at rest 
there is no coloured edge, although, of course, the light is much 
more intense than during rotation. Then as the speed is increased 
the greenish phase appears first at the edges, and it disappears from 
the edges just as it begins to establish itself in the middle. When 
the speed went up very gradually, one could see the coloured edges, 
as it were, breaking up and being diffused over the rest of the image. 
In the last stage coloured edges are never seen. The changes cease 
at the edges a little sooner than in the central part, and never by 
any chance does a coloured edge survive the period of change. If 
they were contrast phenomena, it is hard to see why this should be 
so. With very strong sunlight it is only at the edges that the colour 
