On the extinction of light by an illuminated retina

In “Colour Photometry, Part III,” ‘Phil. Trans.,' 1892, and “On the Sensitiveness of the Retina to Light and Colour,” ‘Phil. Trans.,’ 1897, a discussion on the extinction of light by a “dark-adapted” eye is given. Since the latter date, a large number of experiments have been made by myself and others on the extinction of light when the retina as a whole has been stimulated by illumination of white or coloured light. In this communication the results obtained when the stimulation is by white light are described. In the two papers above referred to the apparatus used when the eye was dark-adapted is described, but for the observations made with an illuminated retina a modification had to be made. A description of one form, which answered as well as any other form, is given. BB is a box as in fig. 1. At the end of the box is cut a hole ¾ inch in diameter and against it, but inside, is placed a 4-inch disc of white matt paper, in the centre of which is cut a ½-inch circular hole. Behind the box is a second end AA separated from the first by a couple of inches. Opposite the aperture at the first end of the box is cut a circular aperture 1 inch in diameter, against which is placed a piece of doubly-ground white glass, and if necessary a second piece can be placed behind it. The ray of the spectrum from the colour-patch apparatus can be reflected from a mirror M on to the ground glasses atd . The 4-inch white disc is illuminated by the white reflected beam of the same apparatus (or by any other light) through an aperture CC cut in the side of the box. This beam partly goes through the aperture at the end of the box and falls on G, a blackened surface, and is completely hidden from the eye end E. At the side of CC a small metal disc can be placed, which casts a sharp black image on the white disc, as shown in fig. 2. This may be taken as a measure of the blackness to be matched when extinguishing the light from the colour.