The optical properties of the ommatidium in the compound eye of the firefly Phausis splendidula L. were investigated. 1. The dioptric apparatus - corneal lens and processus corneae - is composed of layers with different optical density. The highest refractive index is found in the centre of the lens and of the pseudocone (ne=1.520; Kg. 4a). The optical density decreases to the peripheral zone (ne=1.356). The corneal lens is a homogeneous spherical lens, the pseudocone is a lens cylinder (Fig. 5). 2. The refractive index of the cone is ne=1.349, and that of the pigment cells is ne=1.340. The cone acts as a wave guide, the angle of total reflection αT is 83.4°. The optical density of the proximal rhabdoms (Fig. 1) is ne=1.351. 3. Parallel rays entering the facet of an ommatidium converge to one point in the focal plane which lies inside the pseudocone. They leave the process of the corneal lens again as parallel rays (Fig. 5). The way of light beams through the dioptric apparatus corresponds to the theory of Exner (1891). The width of the visual field of a single ommatidium is about 8°. 4. The coni act as wave guides, the whole irispigment does not migrate between the pseudoconi in the dark-adapted eye (Horridge, 1968). Light leaving an ommatidium is captured by the pigment. For these reasons it is uncertain whether a superposition of several pictures, each formed by one ommatidium, occurs. © 1969 Springer-Verlag.
