VISUAL ANALOGUES 173 



exposure, that the immediate after-image would be 

 negative (see below). The relative persistence of 

 after-effect of lights of different intensities may be 

 shown in the following manner : 



If a bold design be traced with magnesium powder 

 on a blackened board and fired in a dark room, the 

 observer not being acquainted with the design, the 

 instantaneous flash of light, besides being too quick for 

 detailed observation, is obscured by the accompanying 

 smoke. But if the eyes be closed immediately after the 

 flash, the feebler obscuring sensation of smoke will first 

 disappear, and will leave clear the more persistent after- 

 sensation of the design, which can then be read dis- 

 tinctly. In this manner I have often been able to see 

 distinctly, on closing the eyes, extremely brief pheno- 

 mena of light which could not otherwise have been 

 observed, owing either to their excessive rapidity or to 

 their dazzling character. 1 



After-oscillation. In the case of the sensitive silver 

 cell, we have seen (fig. 105), when it has been subjected 

 for some time to strong light, that the current of 

 response attains a maximum, and that on the stoppage 

 of the stimulus there is an immediate rebound towards 

 recovery. In this rebound there may be an over-shoot- 

 ing of the equilibrium position, and an after-oscillation 

 is thus produced, 



1 As an instance of this I may mention the experiment which 1 saw on 

 the quick fusion of metals exhibited at the Royal Institution by Sir William 

 Roborts-Austen (1901), where, owing to the glare and the dense fumes, it 

 was impossible to see what happened in the crucible. But I was able to 

 see every detail on closing the eyes. The effects of the smoke, being of less 

 luminescence, cleared away first, and left the after-image of the molten metal 

 growing clearer on the retina. 



