(3) At the opposite end of the plank there is attached a flat board, 35 centimetres long and 30 centimetres wide. Attached to the edge of the board which faces the head-rest is a piece of black cardboard 40 centimetres long by 35 centimetres broad. In the centre of the cardboard is a rectangular aperture, 7 centimetres by 14 centimetres. On the upper surface of the board are two slots, one at either side. Sliding within each of these slots is a block of wood to which is attached an upright sheet of black-painted tin, 15 centimetres wide and 20 centimetres high. The surfaces of these tins lie in planes parallel to the plane of the four wires in the head-rest, when the latter is at right angles to the plank. When their surfaces are equidistant from the wires, the inner vertical edges of the tins are separated from each other by 3 centimetres. The sides of the slots, in which the blocks with their tins slide, are fitted with millimetre scales, thus enabling the experimenter to determine the distance of the edges from the corneæ. The point on the scale at which an edge was exactly 2 metres from the vertical plane of the wires was chosen as the "zero" point, and if this distance was decreased by moving an edge forward, the latter was said to stand at "minus" one, two, or more millimetres, as the case might be. Likewise, an edge was said to stand at "plus" the number of millimetres' distance beyond the zero point if it had been moved at a greater distance than 2 metres from the wires. A piece of ground glass attached to the distal end of apparatus enabled the experimenter to secure a uniform illumination, the room being darkened and the light coming from a 32-candle-power electric lamp set about a metre and a half behind and on a slightly lower level than the glass. It was found that by shading the lamp itself and admitting a dim light to the room by means of drawing down only the ordinary thin window-shades, the edges could be made to seem almost isolated in space and to stand out in clear relief. The subjects of the experiment were Messrs. Bell, Flexner, and Tait. Each subject determined the equality-point and the threshold for the normal primary position of the eyes, for the eyes in a lateral position of 15° and in a lateral position of 30°, both to the left and to the right. Eyes at 0° means the following: that the most anterior part of the two corneæ lies in a plane parallel to and two metres' distance from the plane in which the two parallel edges lie at 0. Eyes at 30° to the left means that a line drawn in front of the two corneæ intersects such a line at an angle of 30°, the left eye being at the distal end of the line. In calculating the visual angles 7.4 mm. are added in order to compensate for the distance from the extreme anterior portion of the cornea to the nodal point of the eye. The results for Mr. Tait are as follows: