VISION. 137 



Fix a pane of plate glass vertically across the middle of a board about 16 

 inches long by 8 wide. On either side place a sheet of paper of the colours 

 which it is desired to blend. Arrange the board so that the illumination of 

 each sheet may be varied, independently of that of the other, and that one 

 sheet is seen through the glass, the other by reflection. 



10. The angle Of rotation (see p. 113). Draw on a wall of a mode- 

 rately dark room, a horizontal line, at a height of a couple of feet above that 

 of the eyes. In a black card, cut out a cross, each bar of which should be 

 about a twentieth of an inch in width. Close one eye, and place the cross 

 between the other and a bright lamp, and fix the eye on the luminous cross for 

 several seconds. Then turn to the wall, which should be at a distance of 

 four or five feet, and direct the eye to a point exactly opposite it and at the 

 same level. If now the eye is fixed on a point in the horizontal line imme- 

 diately above the first (the position of the head being unaltered), it is seen 

 that the transverse bar of the bright image of the cross coincides with the 

 line. But if (the eye remaining fixed) the head is turned to the right, 

 the image gradually assumes an appearance of distortion, the upper end 

 of the upright bar seeming to incline to the left, and the outer end of the 

 horizontal bar to incline upwards. As in reality the horizontal bar coincides 

 with the horizontal meridian line of the retina, it is clear that the retinal 

 image of the horizontal line crosses the meridian line at an acute angle. This 

 angle is the angle of rotation for the particular (tertiary) position assumed by 

 the eye. 



!! Judgment Of Form. For experiments on this subject pairs of 

 diagrams representing respectively the right and left aspects of characteristic 

 objects are used, of which the retinal images are blended by means of the 

 stereoscope. The most important observations are the following : (a) If two 

 diagrams representing the right and left aspects of a pyramid are imaged on 

 the right and left retina, and the images blended by giving the eyes the degree 

 of convergence necessary to unite the apices, a solid pyramid is seen. If the 

 diagrams are transposed and the process repeated the pyramid appears 

 hollow, (b) If two similar diagrams are viewed stereoscopically, of which one 

 is represented white with black edges on a black ground, and the other black 

 with white edges on a white ground, the combined image is lustrous. 



12. Judgment Of Distance. If a number of balls of similar colour, 

 but differing in size, are allowed to fall one after another before one eye, the 

 the other being closed, at such distances that in each case their retinal images 

 are equal, and at such velocities that their images pass over the same retinal 

 distance in the same time, the observer is unable to form any judgment either 

 as to their size, distance, or rate of motion. All of these can be judged of at 

 once if the other eye is opened. For this experiment an apparatus is used. 



