Page:Mind (New Series) Volume 12.djvu/492

478 one or other of two states of consciousness predominates. Now, I find that, in the case of myself and of several other subjects, the periods of complete predominance of one or other form do not make up the whole period of observation, but that between these periods there come others, in the case of two differently coloured fields, periods during which the two colours appear fused or mixed as irregular patches, and in the case of ambiguous figures, such as those of figure 6, periods during which the figure does not assume either of the dominant forms, but appears as a number of lines in one plane of which some may or may not group themselves in consciousness apart from the rest. It seems necessary therefore to use a form of apparatus that will enable the subject to register the duration of these intervening periods as well as of the periods of distinct dominance, and I have devised for this purpose the apparatus described in the note below. A brass cylinder 590 mm. in circumference is made to rotate on a horizontal axis by clockwork driven by a heavy weight. The motion is kept regular by a delicate pair of wind-sails and can be set to any required speed of a considerable range by adjusting the driving weight and the size of the wind-sails. For these experiments the speed was adjusted to give one complete rotation in 118 seconds, which means a translation of any point on the surface of the drum at the rate of 5 mm. per sec. The drum is sheathed in a sheet of smooth white paper of such a length that its two ends united by gummed slips do not quite meet, but leave exposed a slip of the brass cylinder about 4 mm. in breadth. The writing apparatus consists of a tubular glass pen or ink-holder drawn to a point capped with brass. This is clamped in a penholder which forms one arm of a two-armed lever, the other arm carrying an adjustable counterweight. The fulcrum of this lever consists of a piece of small brass tube about 20 mm. in length which slides smoothly upon a steel rod held by adjustable clamps in the horizontal position between two metal upright supports which are carried by a strong iron base-plate. This is placed so that the horizontal steel rod lies parallel to the surface of the drum and at such a distance that the point of the pen lies lightly upon the paper. As the drum rotates the pen then traces a continuous line and after each complete rotation it slips off the edge of the paper on to the narrow strip of exposed metal surface, and in so doing causes the drum to give out a bell-like note which signals to the operator the completion of the rotation. A second pair of uprights fixed in a separate base-plate carries a second horizontal steel rod on which also slides a short piece of brass tubing encased in india-rubber; this is the finger-piece. This second rod carrying the sliding finger-piece is placed parallel to the first and at any distance from it that is most convenient. Attached to one of the supports of the rod carrying the penholder is a small brass pulley, and a silk thread attached to the finger-piece is carried from it round this pulley and tied to the axis of the penholder. This sliding axis is also attached to the support of the other end of the rod on which it slides by a light india-rubber band. The range of movement of the finger-piece is limited to about 25 mm. by a pair of stops on the steel rod, and the length of the silk thread joining