74 Mr. J. S. Dow on the Physiological 



increasing speed the violent flicker tends to fuse into a con- 

 tinuous impression o£ luminosity, and is ultimately replaced 

 by the fine flicker which requires a higher speed to cause it 

 to disappear. Lastly this too disappears. The impulses 

 received through both sets o£ light-perceiving organs may 

 now be supposed to follow so rapidly as to produce a con- 

 tinuous luminous impression. 



A fact recorded by Porter and other observers that the 

 critical speed of disappearance of flicker seems to depend 

 upon the angle subtended at the eye by the flickering surface 

 (e. e., on the portion of the retina on which the image is 

 received), would seem to be very conveniently explained by 

 this view of the action of the two sets of organs. 



An interesting point has been raised by Kriiss *, which 

 seems at first sight against the view that the action of a 

 flicker photometer at high illuminations is mainly based on 

 the behaviour of the cones. This observer points out that, in 

 comparing say a Hefner lamp with an incandescent mantle 

 by means of a flicker instrument, we notice that, when the 

 speed is low and the photometer out of balance, fluctua- 

 tions both in light and colour occur. As the speed is 

 increased to what may be termed the working range the 

 colours fuse into one another, but we still observe a flicker 

 due to fluctuations in brightness, and by the aid of this we 

 balance the photometer. This seems at first sight incon- 

 sistent with the supposition that the action of the instrument 

 is based upon the cones with which the perception of both 

 light and colour are associated. But it must be recalled that 

 the colours of the two sources named are very impure ; we 

 are really comparing not red and green, but white lights 

 slightly tinged with red and green respectively. Now the 

 writer has found that, as the colours of the lights which it is 

 sought to compare become more pronounced, so does the 

 range of speed over which a flicker instrument can be used 

 with success become narrowed down. In the case of very 

 widely divergent colours there is really only one particular 

 speed at which the colour-flicker has just disappeared, and 

 the loss of sensitiveness due to the fusion of the light-impulses 

 has not begun, at which the instrument can be used with 

 accuracy. In short, the fusions of the colour- and light- 

 impulses, when fairly pure colours are used, begin at just 

 about the same speed ; this, therefore, if anything would 

 seem to favour the view that the action of the instrument 



* Journ.f. GasleleucUwiff, xxiv. June 16, 1907, p. 513. 



