Produced hy rapidly rotating IHsks. 139 



In this case the dark stripes form an odd number, and 

 the proportion of white is greater than the black ; hence 

 the rings ; but suppose the proportions of the two colours 

 to be strictly equal ; that the disk is poised exactly on its 

 centre, and very slightly rotated without the slightest 

 eccentricity of motion, then the stripes, radii, chequers, or 

 whatever figures occupy the surface of the disk (except 

 concentric circles) blend into one uniform tint midway be- 

 tween the two colours of the'disk, entirely free from rings. 



From this we may deduce : 



1 . That, when a disk is so divided that with any radius 

 a circle would pass through equal portions of the two 

 colours, an universal blending of colour will result. 



2. That if at any part of the disk a balance of colour be 

 not observed at opposite sides of the centre, concentric rings 

 will result. 



3. That, as the wow-existence of that balance depends upon 

 a definite principle of construction, the number and breadth 

 of the rings can be computed ; but, 



4. If the equipoise be disturbed by extraneous causes, 

 such as imperfect division, or eccentricity of adjustment, 

 the rings are uncertain and incommeasurable. 



But the disk, fig. 2, when employed with the mirror ap- 

 paratus, assumes altogether a different arrangement, of 

 which 



Fig. 2 b. 



will convey an idea. The surface is laid out in curved con- 

 centric bands of the utmost symmetry, the number and 

 breadth of which are the same as in the rectilinear striae. 

 The centre of these concentric segments, which is at the 



