Fig. 1. Illumination of the ventral (A) and dorsal (B) retina. (A) A patch made of black, opaque, vinyl tape was affixed to the ventral cornea using cyanoacrylate glue. The patch was placed over more than half the retina (see side view), to reduce stimulation of the central retina, and the embryonic fissure where ultraviolet corner cones remain resident in all stages of salmonid development (see text). The fibre optic and quartz screen coupler were angled from above the eye, so that the light was incident on the dorsal cornea. The incident light ray shown is not necessarily representative of all light rays emerging from the fibre optic coupler; it is positioned for illustration purposes only. Light rays incident on the lens from above are refracted by the lens such that light will illuminate an area on the ventral retina. Because the stimulus was larger than the pupil of the fish, the configuration shown would result in illumination of the entire ventral retina. (B) A complementary arrangement of the patch and stimulus allowed for illumination of the dorsal retina.