114 SCIENTIFIC RECREATIONS. 



line of separation of the two colours ; the piece of grey then stretching into 

 the green will present as deep a red as the red foundation on the other side. 

 If the line of grey colour exactly covers the line of separation between the 

 two colours, the contrasting colour is more feeble ; the edges of the grey 

 paper then present complementary strips of colour. Similar effects may be 

 obtained by superposing, in gradually diminishing layers, strips of thin paper, 

 so as to form successive bands of different thicknesses. If it is then lit 

 up from behind, the objective intensity is evidently constant through the 

 extent of each layer ; nevertheless every strip appears darker at the edge 

 touching a more transparent layer, and lighter at the edge in contact with a 

 thicker layer. The dull tints of China ink, superposed in layers, will produce 

 a similar effect. The phenomena are produced by means of rotative discs 

 of most beautiful and delicate gradations of colour. Let us give the sectors 

 of the disc the form represented by fig. 114, and make them black and 



Fig. 114. Disc, which exhibits, when in rotation, a series of concentric rings. 



white ; and when in rotation we shall see a series of concentric rings of a 

 shade that becomes darker and darker towards the centre. The angular 

 surface of the dark portions is constant in each of these rings. The intensity, 

 therefore, of each ring is uniform during rapid rotation ; it is only between 

 one ring and another that the intensity varies. Each ring also appears 

 lighter on its inner side when it borders on a darker ring, and darker on its 

 outer side when in contact with a lighter ring. If the differences of intensity 

 in the rings are very slight, one can scarcely judge sometimes if the inner 

 rings are darker than the outer ; the eye is only struck by the periodical 

 alternations of light and shade presented by the edges of the rings. If, 

 instead of white and black, we take two different colours, each ring will 

 present two colours on its two edges, although the colour of the rest of the 

 ring will be uniform. Each of the constituent colours presents itself with 

 more intensity on that edge of the ring which borders on another ring 

 containing a smaller quantity of the colour. Thus, if we mix blue and 

 yellow, and the blue predominates in the exterior and the yellow in the 



