The angle between optical axes of adjacent photoreceptors in a single ommatidium of Lethocerus has been measured by optical methods and confirmed by electrophysiological recording and the optical system by which this comes about has also been demonstrated by tracing rays through a model eye in which the relevant optical constants have been determined. In the dark-adapted anterior part of the eye, in air, the inclination between rhabdomere axes within a single ommatidium is approximately 10 degrees but the interommatidial angle is 2.5 degrees -3.0 degrees. Therefore each receptor sees the same field as a particular pattern of other receptors 3-4 facets away in each direction. Although adjacent rhabdomeres lie side by side, they have separate fields of view, which reach 10 degrees wide at the 50% contours in the dark-adapted state. In the light-adapted eye the cone extends as a crystalline tract between the corneal cone and the rhabdomeres. The fields of view of all receptors of an ommatidium now coincide. The changes upon adaptation imply an interesting problem as to how the projection of the outside world can apply to both the light-adapted and the dark-adapted eye. Apparently cells 7 and 8 form a photopic system which retains acuity in both states of adaptation, and cells 1-6 are functional only as a high-sensitivity large field scotopic system.