Produced hy rapidly rotating Disks. 141 



These chequers, when viewed through a slit, parallel to 

 the rectilinear division, present 



Fig. 3 a. 



Y 



evidently partaking in the general character of fig, 2 b, the 

 difference being due to the division of the striae into squares. 

 If the slit be inclined 45° to the divisions, all the diverging 

 lines proceeding from Y, figure 3 a, cut the diagonals of 

 the chequers instead of marking their boundaries. 



The same principles obtain through an endless variety of 

 figures : thus, if the disk be diametrically divided into two, 

 and the slit be on the line of division, the figure is alto- 

 gether unchanged ; but if it be at right angles to that line, 

 the half which contains the slit encroaches upon the other 

 half by the curvature of the line of division, thus presenting 

 the gibbous form of a three quarter moon. 



Nearly the whole of the experiments now detailed were 

 performed with the mirror-apparatus, for the purpose of 

 getting uniformity in the revolutions, otherwise no definite 

 figures could have been given ; but the figures are the same 

 in principle as those produced by two revolving disks, the 

 latter arrangement presenting figures modified in every 

 imaginable way by one or more of the five conditions 

 before stated. 



When two disks, for instance, are moving with unequal 

 velocities, the nucleus towards which the curves tend, or 

 from which they seem to spring, oscillates to and fro, and 

 the curves themselves vary in their number and respective 

 distances from each other. 



The slitted disk is a modification of M. Plateau's Fan- 

 tascope, the latter containing many short slits, and the 



