SENSATION. 23 



on a glass pane, which rests ou four supjDorts ; under the 

 paper put the first gray scrap. By means of a wire, fasten 

 the second gray scrap 2 or 3 cm, above the glass plate. 

 Both scraps appear exactly alike, except at the edges 

 Gaze now at both scraps, with eyes not exactly accommo- 

 dated, so that they apj)ear near one another, with a very 

 narrow space between. Shove now a colored field (green) 

 underneath the glass plate, and the contrast appears at 

 once on both scraps. If it appears less clearly on the 

 upper scrap, it is because of its bright and dark edges, its 

 inequalities, its grain, etc. When the accommodation is 

 exact, there is no essential change, although then on the 

 upper scrap the bright edge on the side toward the light, 

 and the dark edge on the shadow side, disturb somewhat. 

 By continued fixation the contrast becomes weaker and 

 finally yields to simultaneous induction, causing the scrajDS 

 to become indistinguishable from the ground. Remove 

 the green field and both scraps become green, by succes- 

 sive induction. If the eye moves about freely these last- 

 named phenomena do not appear, but the contrast continues 

 indefinitely and becomes stronger. When Helmholtz found 

 that the contrast on the lower scrap disappeared, it was 

 evidently because he then really held the eye fixed. This 

 experiment may be disturbed by holding the upper scrap 

 wrongly and by the difierences in brightness of its edges, 

 or by other inequalities, but not by that recognizing of it 

 'as an independent body lying above the colored ground,' 

 on w^hich the jasychological explanation rests. 



In like manner the claims of the psychological explana- 

 tion can be shown to be inadequate in other cases of con- 

 trast. Of frequent use are revolving disks, which are 

 especially eflicient in showing good contrast-phenomena, 

 because all inequalities of the ground disappear and leave 

 a perfectly homogeneous surface. On a white disk are ar- 

 ranged colored sectors, which are interrupted midway by 

 narrow black fields in such a way that when the disk is re- 

 volved the white becomes mixed with the color and the 

 black, forming a colored disk of weak saturation on which 

 appears a gray ring. The latter is colored by contrast with 



