364 Mr. Shelford Bidwell [March 5, 



ness is much more than gV ^^^^ ^^ ^ ^^' ^^® lines, as seen by an 

 observer at a distance of two or three feet, do not become red through- 

 out 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 disc some designs in black lines and 

 letters were made to appear red, the effect being visible in all parts of 

 the theatre.] 



When the disc is turned in the opposite direction, the black lines 

 appear at first sight to become dark blue. Attentive observation, 

 however, shows that the aj^parently 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 phe- 

 nomenon which seems to present itself when a dark patch is suddenly 



Fig. 9. 



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. 



A circular aperture was cut in a wooden board and covered with 

 white paper : a lamp was placed behind the board, and thus a bright 

 disc was obtained as in the former experiment. An arrangement was 

 prepared by means of which one half of this bright disc could be 

 suddenly covered by a metal shutter, and it was found that when 

 this was done a narrow blue band appeared on the bright ground just 

 beyond and adjoining the edge of the shutter when it had come to 

 rest. The blue band lasted for about -^^ second, and it seemed to 

 disappear by retreating into the black edge of the shutter. An 

 attempt has been made to illustrate it in Fig. 9, where the shaded 

 band indicates the blue border. 



