In studying the struggle of two differently coloured visual fields with the aid of the apparatus described in the note I sit before a prism-stereoscope of the ordinary form, fixed in a horizontal position over an aperture in the window-shutter of the dark-room, the two fields being patches of light transmitted through coloured paper or gelatine, and I hold between the finger and thumb of the left hand the finger-piece of the apparatus described in the note. During complete predominance of one coloured field I hold the finger-piece against one stop and on predominance of the other I slide it over and hold it against the other stop and when the two colour-fields appear fused or mixed in patches I hold it in a position mid-way between the stops. In this way a line of the form of figure 6 is traced on the paper and it constitutes a record of the number of changes of colour and of the duration of each phase of the struggle during the 118 sees, of rotation of the drum.
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Fig. 6.
In this way I have made a considerable number of series of observations, but since I do not think that the average of the results of a large number of such series will give a truer picture than the records of a single one of the most satisfactory and representative series, I will present the record of one such series, merely stating that it is in harmony in all important points with the other records obtained in similar experiments. In this series the competing fields were two small squares (2 mm. wide but slightly magnified by the stereoscope), either one occupying the centre of a white square of about equal brightness and 10 mm. in width. These two fields were mounted on separate black cards, the light of the two square fields being transmitted through holes in the cards, and the rest of the aperture in the shutter
the two sliders is so adjusted that the finger-piece is held against the one stop by the tension of the rubber band. The pen-holder then accurately reproduces every sliding movement of the finger-piece between its two stops, and the pen records these movements upon the moving paper. This apparatus is very simple and constant in the working, it involves neither electrical connexions nor smoked surfaces, both of which are apt to prove unmitigated nuisances, and it can easily be worked by the subject without the aid of an assistant.
