of Simultaneous Contrast- Colour. 



157 



paper of A there is a more intense orange, and when the blue 

 of the translucent paper of C overlaps the blue of the ring of 

 A we have a more intense blue. On bringing B of the trans- 

 lucent paper of over Y of the translucent paper of A we 

 have white. 



Experiments on the Complementary Colours of Gratings. — 

 Out of thin cardboard, such as is used for thin visiting-cards, 

 I cut gratings with a dividing-engine (see fig. 6). The widths 

 of the spaces cut out of these gra- 

 tings exactly equalled the breadths 

 of the cardboard left in the grating. 

 Gratings were thus made having 

 spaces of 1, 2, 3, 4, and 5 millim. 

 The cardboard I rendered opaque by 

 coating one side of it with ivory- 

 black in dilute shellac-varnish. 

 After it has dried, the cardboard is 

 well flattened before it is fastened 

 to the surface of a piece of hard 

 wood on the dividing-engine. The 

 cutting-edge of the cutter for this 

 work must have a very acute angle, 

 down a rod of Stubs' steel. 



Fi<?.6. 



I made one by grinding 

 Heating this to a dull cherry-red 

 and then forcing it into a large ball of beeswax gives the edge 

 of the cutter the required temper, without the necessity of 

 subsequently " letting it down." The blackened side of the 

 gratings was covered with the 

 "Alba tracing-paper." The 

 grating was mounted back of 

 an opening in a black card- 

 board-screen, so that only the 

 white grating was exposed. 



Two other black screens, Wh O \* 



and Y of fig. 7, having open- 

 ings of the same size as the 

 grating, and covered on the 

 back with the translucent 

 paper, were placed on either 

 side of the grating, G. In 

 fig. 7, L is the lamp. The 

 window is on the other side 

 of G. The screen, S, divides 

 the apparatus so that the light 

 of the window, W, reflected 

 by the mirror, M, on to the 

 screen, Wh, cannot fall on 

 illuminated by the lamp L. 



\vk. 



G- 



Rg.7. 



G or on Y, which are only 



