Produced by rapidly rotating Disks. 



137 



semi-diameter in the centre of one of the bands, and it 

 may be stated, once for all, that the eye is placed behind 

 the disk, so that the front of the disk may be seen re- 

 flected in the mirror, which occurs once during one 

 revolution of the disk, and if the disk perform more 

 than six revolutions per second, an uninterrupted view of 

 the figure can, of course, be obtained. This arrangement 

 may be termed the mirror apparatus. 



Under these circumstances figure 1 assumes the form 

 Fig. 1,«. 



Fig. 1 a. 



Which offers a general view of the figure, for the curvature 

 of each radius varies with every position of the eye, con- 

 sidered with reference to the inner or the outer end of the 

 slit. When the eye is near the inner end of the slit the 

 radii are congregated much nearer the point A., (figure 1 a) 

 while at and about B the interval between the two upper 

 radii is greatly increased ; but if the eye be held near the 

 outer end of the slit the radii are less curved, and their dis- 

 tribution more equable ; but at whatever part of the slit the 

 eye be applied this rule is constant — that the point A, to- 

 wards which the curves tend, is seen at the axial end of the 

 slit, in consequence of the slit being only at one side of the 

 axis, and the radial band occupying that positiqn is of the 

 same colour as that which contains the slit. If, however, 

 the slit, instead of occupying the centre of one radial band, 

 be on a line of division between two bands, a line of division 

 will occupy the central position, which in the former case 

 was occupied by a band, and, in the latter case, the bands 

 a, b, c, d, will be opposite in colour to e,/, g, h. 



