316 



Mr. T. C. Porter. 



[May 14, 



passage of the image of the white and black sectors of the disc over 

 the points of the retina, and will be equivalent, so far as the effect 

 upon nicker is concerned, to making the disc rotate more slowly. 

 There are two kinds of motion the eye may make without causing the 

 nicker to reappear — the one radial, the other round the disc, in the 

 opposite sense to that in which it is rotating. Experiment verifies these 

 conclusions, though in practice the radial movement is not very easy 

 to accomplish. 



A set of experiments was then made, paying due regard to all the 

 foregoing precautions, to determine the exact number of revolutions of a disc 

 half white, half black, at which flicker just vanishes, when the rotating disc is 

 differently illuminated. 



The experimental arrangements can best be understood from fig. 1. 



Fig. 1. 



I— Id 



s d I e- 



A E 



S is the source of the illumination of the disc D : this illumination can 

 be changed in two ways, viz., either by altering the distance of D 

 from S, or by changing the illuminating power of S itself. The length 

 of the bench was more than 5 metres, and the illumination of the disc 

 varied from that given by a sperm candle burning 8*273 grammes, or 

 127*67 grains per hour (which is meant when 1 candle-power is men- 

 tioned in this paper), at a distance from the disc of 4 metres, to the 

 illumination given by an arc light of approximately 1600 ( ± 50) candle- 

 power at a distance from the disc of J a metre. 



Returning to the figure, LL PP are inner and outer boxes, painted 

 with dead black inside, having openings at and 0', with sharp edges, 

 carefully blackened, wide enough to allow the whole of the illuminant 

 to be seen from every part of the disc, when the latter was at its 

 nearest to the illuminant. The screens PP LL kept the light from S 

 from falling on the walls and ceiling of the room, and care was taken 



