102 ON THE RELATION OF OPTICS TO PAINTING. 



it appears to the eye to be covered with three grey 

 rings as in Fig. 4. The reader must, however, figure 

 to himself the grey of these rings, as it appears on 



FIG. 3. FIG. 4. 



the rotating disk of Fig. 3, as a scarcely perceptible 

 shade of the ground. When the rotation is rapid 

 each ring of the disk appears illuminated, as if all the 

 light which fell upon it had been uniformly distributed 

 over its entire surface. Those rings, in which are the 

 black bands, have somewhat less light than the quite 

 white ones, and if the breadth of the marks is com- 

 pared with the length of half the circumference of the 

 corresponding ring, we get the fraction by which the 

 intensity of the light in the white ground of the disk is 

 diminished in the ring in question. If the bands are all 

 equally broad, as in Fig. 3, the inner rings appear darker 

 than the outer ones, for in this latter case the same 

 loss of light is distributed over a larger area than in 

 the former. In this way extremely delicate shades of 



