The first image in the regular series was reddish white with dark lines. The colour gradually deepened, and the red negative image appeared, followed after several fluctuations by a dark blue negative image, very bright and distinct. The blue and green positive stages were regularly omitted when the sky was darkened; and the effect noted above, the brightened green which appeared several times when the subject tried to turn the image red, was very likely due to a brightening of the sky, as it was not observed in the later experiments, which were never made when the sky was in a condition to render the illumination variable in any marked degree.
When the subjects were sufficiently practised to be familiar with the ordinary course of the image and to find it regular, they were told before the experiment that they must by an effort of will turn the image red all the way through its course; that they must force back the other colour stages, or make them as brief as possible by fixing their attention on the idea of red. It was suggested that this could be brought about best by thinking hard of something red, at the same time trying to call up the colour in the retinal field; by fancying, for instance, that the window-panes were pools of blood. Corresponding suggestions of green and blue were made in other experiments; the subjects being told to think of green grass or blue sky. It soon became apparent that the carrying out of these suggestions was much easier for one of the subjects (D.) than for the other three. While M., W. and O. have moderate visualising powers, D. is a good instance of the predominantly visual type, and was selected as a subject for that reason. In the case of the former, intense effort was needed to keep the attention fixed on the colour to be produced. This effort was manifest in a rigid position of the body, in frowning and biting the lips, and in a general expenditure of muscular energy, which became very fatiguing. The subjects often tried to reinforce the visual excitation by associated sound and muscular excitations, repeating internally the words 'red' or 'green' while trying to call up the colours. The strain of all this was found by the writer to be so great that the conditions of the experiment were altered, and instead of being required to visualise one colour throughout the whole course of the image, the subjects were asked in one set of experiments to turn the image red, blue, or green during the first half of its course, that is, up to the point where it passed from the positive to the negative stage; and in another set to wait until the image was entering upon the
