1. Intracellular recordings from single retinula cells in the locust compound eye show that the visual field of a single cell is wider in dark-adapted than in light-adapted eyes. 2. The time course of the increase in the visual field of a single cell is correlated with the replacement of a dense packing of mitochondria by a fluid filled palisade around the rhabdom during dark-adaptation. 3. The development of the palisade results in a reduction of the refractive index immediately around the rhabdom. 4. The correlation between the increase in the visual field of a single cell and the development of the palisade is evidence for internal reflection in the rhabdom. 5. Direct tests of the acuity of single retinula cells to equal black and white stripes of various repeat distances show reasonable agreement with theoretical considerations based upon Gaussian acceptance curves, and is dependent upon the state of adaptation of the eye.