374 Mr. S. Bid well. On Subjective Colour Phenomena 



equal parts by a straight line from the centre to the circumference, 

 and one of these parts was painted black. The disk was attached to 

 a horizontal spindle, turned by a motor at the rate of five or six 

 revolutions per second, while its front was illuminated by a lamp of 

 16-candle power. A white card, upon which was a black line, or a 

 design composed of black lines, was supported behind the disk, and 

 viewed intermittently through the open sector. When the rotation 

 was such that the open sector succeeded the black portion of the 

 disk and was succeeded by the white portion, the black lines became 

 red. 



This experiment is identical with the last, except that the white 

 ground is illuminated entirely by reflected light. In conjunction 

 with the others, it indicates with certainty the origin of the remark- 

 able red colour shown by Benham's top. 



The disk with the open sector affords a mucli more convenient 

 means than the top of exhibiting the colour phenomena. If a disk 

 with an open sector of 45 or 60 is made of white cardboard, and a 

 movable black half disk is mounted in front of it upon the same axis, 

 we may, by suitably adjusting the position of the black half disk 

 with regard to the opening, produce in a fixed object all the tints 

 shown by the top, as well as intermediate ones ; .and the object itself 

 may be easily changed to suit the conditions of an experiment. 



Experiment VII. 



If the commutator of Experiment V, or the disk with the open 

 sector of Experiment VI, be turned in the reverse direction, the 

 strips of tinfoil or the black lines appear to become blue (instead of 

 red), like the outer group of lines in Benham's top when it spins in 

 the direction indicated by the arrow in the figure. This appearance 

 is partly, if not altogether, illusory. It is the bright ground in the 

 immediate neighbourhood of the black lines that becomes blue ; the 

 lines themselves (except possibly just within their extreme edges) 

 become a neutral grey, owing to the alternations of light and dark- 

 ness or of white and black. 



A card with some black lines 1 mm. thick drawn upon it was 

 placed behind the disk with the open sector of Experiment VI, which 

 was turned in the directiou such that the open sector was pre- 

 ceded by white and followed by black. The lines presented the 

 appearance of having been drawn with blue ink upon imperfectly 

 sized paper, a blue stain having apparently spread for a short dis- 

 tance on both sides of the lines. 



Lines of gradually increased thickness were successively employed 

 until at last they had the form of bands f -in. wide ; and even in this 

 latter case it was not easy to see that the bands themselves did not 

 become blue, but only their outlying borders. 



