'Rood [note: 'American Journ. of Science, XLVI. p. 173. 1893.'] has recently re-introduced to notice a method of photometry advocated on not very convincing grounds by Schafhäutl [note: 'Universal Vibrations-photometer: Akad. d. Wiss. München, VII. p. 465.'] forty years ago. The method is based on Plateau's [note: 'Dissert. sur quelques propriétés des impressions &c. Liège, 1829.'] research, and proceeds on the well-known relation existing in light-sensations between their intensity and the rate of frequency of repetition required to fuse them.' (34)

'When the eye is subjected to an alternation of stimuli of a frequency below a certain limit, the sensation of flicker with the accompanying inference of a discontintuous source of light, is produced.

Schafhäutl [note: Akad. d. Wiss. München VII. p. 465. 1855.'] was the first to note that the limit depended upon the strength of stimulus and utilised the method as a means of measuring luminosity.' (396)