1. On the Gradual production of Luminous Impressions on the Eye. Part II

Extract

The author, in a paper communicated to the Royal Society of Edinburgh in 1849, described a method of observation by which he had succeeded in measuring the brightness of visual impressions of short duration. This consisted in causing a disc, with a sector of a known angle cut in it, to revolve with a known uniform velocity between the eye and a luminous object. At each revolution of the disc a flash is seen. The time during which the light has acted on the eye is easily computed from the angle of the sector and velocity of the disc; and the brightness of the flash is ascertained by photometric arrangements. By this method the brightness of impressions formed on the eye by light acting for short intervals of time varying from ·1 to ·001 of a second was ascertained with results which have been described in the paper already referred to.