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A new electrophysiological optometer has been developed, based on Scheiner's principle. Using two light-emitting diodes driven in counterphase, and grating stimuli in Maxwellian view, the system has been used to refract the photoreceptor plane of the pigeon eye. When a grating is conjugate with the photoreceptors, the electroretinographic response (e.r.g.) is minimal, and the image is stationary. As defocus is introduced, so image shift occurs, and the e.r.g. rises on each side of the refractive minimum. Two experiments were devised to test whether a derived minimum point is primary, by using gratings of bar width 3, 5 and 7 units, and by using random checkerboard stimuli. Ray tracing was used to compute the lobe width of dioptric profiles for gratings of spatial frequency 1.47, 0.88 and 0.63 cycles/deg. The computed lobe widths agree well with experimentally determined e.r.g. profiles. Examination of the lateral visual field, on the horizon, shows that the pigeon eye is emmetropic, and well corrected for astigmatism.