VISUAL ANALOGUES 173 
exposure, that the immediate after-image would be 
negative (see below). The relative persistence of 
after-eflect of lights of different intensities may be 
shown in the following manner : 
If a bold design be traced with magnesium powder 
ona 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 I saw on 
the quick fusion of metals exhibited at the Royal Institution by Sir William 
Roberts-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. 
