136 VISION. 



6. Duration and culmination of light sensations. On a 



black card draw two concentric circles, of which the respective diameters are 

 6 and 10 inches. Draw a straight line through the centre, so as to divide the 

 annular space between them into two equal parts. Cover one of these spaces 

 with white paper. Cut out the card along the outer circle and fix it to the 

 revolving disk. 



If the rate of revolution is gradually increased, the moment can be deter- 

 , mined at which the sensations due to successive exposures of the white 

 sector become blended. It will be found that this happens when the rate 

 of revolution is such that the white is visible each revolution for from o"'i5 

 to o" '2 ; for the time required for the light given off by a white surface in 

 common daylight to produce its full sensational effect, is about a sixth of a 

 second. If the rate of revolution is further increased, the subjective lumi- 

 nosity diminishes, but finally becomes constant. Its brightness is then just 

 half of that of the white paper at rest. 



7- Diminution of sensational effect in continued exci- 

 tation Of the Retina. To prove that when the eye is exposed to the 

 light from a bright surface, the apparent luminousness of the surface after 

 culminating gradually diminishes, fix against a wall a black sheet of paper 

 with a small white square in the middle, and place beside it a white sheet of 

 similar size. Having fixed the eye steadily on the white square, suddenly 

 direct it to the adjoining white surface. A grey square is seen on a white 

 ground, of which the shade differs according to the number of seconds that the 

 white square has been contemplated. 



. 8. Smallest perceptible difference. Prepare a piece of black 



paper," six centimeters in length, and varying in width from 2 millims. to 

 8 millims. Cut it transversely into six bits, and apply the smallest to a white 

 disk, half way between centre and circumference, with its long edge against 

 a diameter of the disk. Set the disk in rapid revolution and observe the 

 effect. Replace the bit of black paper by the one next it in width, and repeat 

 the observation. Proceed in this way until a faint grey ring is seen, when the 

 disk is in revolution. This happens when the width of the black surface is 

 about one hundredth of the circumference of the ring. 



9. Visual perception Of Motion. When a disk on which a num- 

 ber of concentric spirals at equal distances from each other are inscribed, is 

 contemplated in rapid revolution, radial motion is perceived, which is centri- 

 petal or centrifugal, according to the direction of rotation. If the eye is sud- 

 denly directed to a blank surface, radial motion is still for a time perceived, 

 but it is in the opposite direction. This experiment serves not only to illustrate 

 the principle enunciated on p. 97, but to prove that the subjective perception 

 is not due, as has been supposed in other similar cases, to felt motions of the 

 eyeballs. 



For experiments relating to the blending of sensations of colour (p. 1 12), 

 disks are used, each of which has a radial cut extending from the circular hole 

 in the centre, to the circumference. Two or more of these cardboard disks 

 can be fixed to the brass disk, in such a way that a sector of each colour may 

 be exposed, and that their relative areas may be varied at will. For many 

 purposes of study, the following method of blending is more useful : 



