712 Messrs. H. E. Ives and E. F. Kingsbury on the 



diagram the light from each — the fluctuations in the light 

 being of much smaller amplitude than in the power supply, 

 because of the thermal capacity of the filaments. Suppose 

 the two lamps to have equal candle-power, but as above, a 

 different character of filament. The illumination conditions 

 on the test-surface at various times due to the two lamps are 



shown in 



fig. 



4, where the line " C " refers to the thick 



Fig. 4. 



Fluctuation in intensity of two incandescent lamp filaments of same 

 mean candle power, but different thickness. C, thick fiiament ; A, thin 

 filament ; S, summation. Their fluctuating sum exhibits also the result 

 of alternating the two equally bright lights represented by a and c in 

 fig. 1. 



filament and the line " A " to the thin filament. The re- 

 sultant of the two (the summation) is obviously a flickering 

 illumination, although the mean value of each component 

 illumination is the same. 



Before carrying this physical case any further let us refer 

 to fig. 1. Here any point on one of the straight lines indi- 

 cates the illumination at which, for that particular colour, a 

 certain speed of alternation causes flicker to disappear. Now 

 the disappearance of visible flicker may be taken as occurring 

 when in the physiological transmitting apparatus the per- 

 centage range of the fluctuation (which has been very much 

 smoothed out, as compared with the original stimulus) reaches 

 a certain small constant value, say one per cent. Any lower 



