206 



SOME CURIOSITIES OF VISION. 



Fig. 8. 



it into two equal parts by a straight line through the center, and paint 

 one half black. At the junction of the black and white portions cut 

 out a gap which may conveniently be of the form of a sector of about 

 45°. (See fig. 8.) Stick a long pin through the center and hold the 

 arrangement by the pointed end of the pin a few inches above a printed 

 page near a good light. Make the disk spin at the rate of about five or 



six turns a second by striking the edge with 

 the finger. As, before, the letters, when 

 seen through the gap, will appear red, and 

 persistence will render the repeated impres- 

 sions almost continuous. Care must be 

 taken that the disk does not cast a shadow 

 upon the printing, and that the intensity of 

 the illumination is properly adjusted. I 

 have here several rather more elaborate 

 contrivances for making disks rotate. 



In none of these experiments does an 

 extended black surface ever appear red, but 

 only black dots or lines, which may, of 

 course, have the form of letters. And the lines must not be too thick; 

 if their thickness is much more than one twenty-fifth of an inch, or 1 

 millimeter, the lines, as seen by an observer at a distance of 2 or 3 feet, 

 do not become red throughout, but only along their edges. The red 

 appearance is, in fact, not due to the black lines themselves at all; these 

 serve merely as a background for 

 showing up the red border which 

 fringes externally the white portions 

 of the paper, and the width of this 

 border does not exceed about one- 

 fifth of a degree. 



(By means of a large rotating disk 

 some designs in black lines and let- 

 ters were made to appear red, the 

 effect being visible in all parts of the 

 theater.) 



When the disk is turned in the 

 opposite direction, the black lines appear at first sight to become dark 

 blue. Attentive observation, however, shows that the apparently blue 

 tint is not formed upon the lines themselves, as the red tint was, but 

 upon the white ground just outside them. This introduces to our notice 

 another border jmenoinenon which seems to present itself when a dark 

 patch is suddenly formed on a bright ground, for that is essentially what 

 takes place when the disc is turned the reverse way. I made some 

 attempts to obtain more direct evidence that such a dark patch 

 appeared for a moment to have a blue border, and after some trouble 

 succeeded in doing so. 



Fig. 9. 



