Mr. T. C. Porter. 



FIG. 2. 



fiw.perse^. Note for 24 holes in disc, 



Ye/tow. - G(768> 



Greenish-Yellow. - F 



- te/lom'sh-Green. ~E 



22, 



te 



-I8-B 



-D 

 Blue-Green. - c# 



-Blue. -A* 



anything, nioie clearly ; the yellow and the violet the two extremes 

 so far as the pteseut considerations go are placed at the top and 

 bottom of the central vertical line, and the number of rotations per 

 second necessary for flicker just to vanish, given in the column on 

 the left, is seen to be (for this half-white, half-black disc) 32 for 

 the yellow, and 16 for the violet, and 24 for the last pair of 

 colours crimson and full green. The comparative rates of rotation 

 for these colours thus bear the ratios of 2 : 3 : 4, which are easily 

 remembered. 



To return to fig. 1. The curves II and IX lie considerably 

 above the others : they are the expression of observations made, 

 as suggested to Professor Rood by Dr. Woolcott Gibbs, by 

 viewing the spectrum through a rotating disc having a sector 

 of 180 removed, using a telescope. Though the form of these 

 two curves is practically the same as that of the others, their 

 higher position, due to the superior intensity of the light received 

 by the eye in this direct vision method, proves clearly what is already 

 well known for white light, that the speed necessary for the disap- 

 pearance of flicker increases with increase of the intensity of the 

 light, whatever its colour may be. No. II is for lime-light, No. IX 

 for sunlight, the rays of the sun being reflected through the slit by 

 means of a heliostat. 



Of the other curves in fig. 2 not much need be said ; they are all 

 for lime-light, the spectrum being thrown on the rotating cardboard 

 disc half white and half black. The writer has thought it better not 

 to smooth the curves in any way. 



These curves, then, not only confirm the already known fact that 



