| Description | Three diagrams in black ink, labelled figures 1-3, showing Gorham's pupil photometer. The instrument is 'a tube two and a half inches long and one and a half in diameter. It is open at the ocular end and closed at the other by a thin circular metallic disc on which are described twenty four equidistant rays. Two punctures are next made in each ray with a small needle at definite intervals [...] a scale of all the intervals is marked on the box [tube] in decimal fractions each fraction corresponding to a given interval. In order to see but one ray at a time the box has a revolving lid with a narrow slit through which any ray may be examined during its revolution.'
Subject: Scientific apparatus and equipment / Optics |