'it was on the above theory [of 'persistance' relating to the phenomena of loss of flicker sensation] that Charpentier [note: 'Compt. Rendus Soc. Biolog. Series VIII. Tom. IV. 1887. (Several papers.)'] in 1887 drew the conclusion from observation on intermittent retinal stimulation that "the persistence of the impressions of luminosity decreases when the light increases and vice versa."

Contrast the statement with that of Helmholtz [note: 'Phys. Optik. p. 503. 1896.'], "the greater intensity of the primary light the greater is the brightness of the positive after-image and the longer does it last."

If curves were plotted to represent the relation of intensity of primary light to the duration of after-image, the intensity being the abscissa and duration the ordinates, upon Helmholtz's view the curve would start from the zero point and gradually rise, the exact nature is not known as direct determination of length of after-image offers insuperable difficulties: according to Charpentier with very small luminosity we should have an after-image of long duration which would gradually decrease as the luminosity increased, in fact the one curve would be the mirror image of the other.' (445)