Contributions to the Study of " Flicker" 355 



ing to the half-and-half disc, but not completely so, for the con- 

 sideration just mentioned would cause the curves which are repre- 

 sented in fig. 3 to be steeper on the right than on the left, viewing 

 the curves from the axis of Y. It will be seen at once that in prac- 

 tice the converse is true, and the writer believes that this is due to 

 the fact that the black sector is not completely black. The effect of 

 the small percentage of light reflected from the black sector will be 

 at first to diminish the rate of growth of the speed necessary for 

 flicker to vanish, for it diminishes the contrast between the coloured 

 sector and the black a contrast on which the flicker primarily 

 depends. The curves will therefore rise more gradually on the 

 right, holding the figure as already described. When there is little 

 of the black left, there will be proportionately little of the light 

 reflected from it, and if this light be bright enough to have any appre- 

 ciable effect, the effect must be to make the decrease of the speed of 

 rotation (necessary to cause the flicker to all but reappear) more rapid, 

 because it lessens the effect of the narrow black sector left, giving 

 the impression that all the coloured sector's light has survived the 

 passage of the black sector, for a rate of rotation at which, in reality, 

 a part failed to survive, and would have produced flicker if unaided 

 by the light reflected from the black sector. 



Thus any want of blackness in the black sector will have con- 

 siderably more effect during the early part of the growth of the 

 coloured sector, whilst the black sector is very large, than afterwards, 

 and this completely explains the observed departure from symmetry 

 in the curves constructed from actual observation. It should be 

 noted that from the way in which the disc is illuminated by the 

 spectrum, any light reflected from the black sector is of the same 

 colour as that of the bright .sector. If pigment had been used to 

 colour the bright sector, and the disc viewed in white light, white 

 light would have been reflected from both the coloured and the 

 black sector, and the effect of this would be very much harder to 

 explain. 



Five curves will be seen in fig. 3 ; they are for different colours of 

 the same lime-light spectrum ; each of the capital letters gives the 

 exact result of an observation, carefully verified in every case. The 

 actual number of rotations per second can be easily found for any 

 point on the curves by dividing the number of vibrations of the cor- 

 responding note found on the axis of X by 24, the number of holes in 

 the syren disc. The musical intervals, so far as the diatonic scale is 

 concerned, are the true chromatic, and not the equal temperament 

 system. 



The information conveyed by the position of any point on one of 

 the curves may be stated as follows, taking, for example, (r on the 

 160 line of the yellowish-green : 



