Stephen W . Lehmkuhle

1. A. Rochon-Duvigneaud, Les Yeux et la Vision des Vertebres (Masson, Paris, 1943). 2. S. L. Polyak, The Vertebrate Visual System (Univ. of Chicago Press, Chicago, 1957). 3. G. L. Walls, The Vertebrate Eye (Cranbrook Institute of Science, Bloomfield Hills, Mich., 1942). 4. R. Shlaer, Science 176, 922 (1972). 5. Gratings of different manufacture, (i) engineruled gratings (Ronchi rulings) and (ii) film copies of the engine-ruled gratings, were used. The film gratings were made in order to provide a wider assortment of spatial frequencies. Highcontrast film (Kodalith ortho film, type 3) was directly exposed to magnified Ronchi rulings projected by an enlarger, a method that yielded precise control of the number of lines imposed on the film. All gratings, both film and engineruled, were examined under a microscope to ensure that contour edges were sharp and that the opaque contours were completely filled. Contrast (Imax Imin/Imax + Immn) measured by a microphotometer was 99+ percent for all gratings. The transparent contours of the engineruled gratings passed 7 to 9 percent more light than those of the film gratings. This difference in transmission was corrected by filtering the engine-ruled gratings. As a test of the equivalence of film gratings and engine-ruled gratings, identical spatial frequencies of both kinds were used in different testing sessions; no differences in discrimination performance were found between sessions for any of several frequencies compared in this way. The grating and the blank stimulus were transilluminated by collimated light from two tungsten-halogen lamps. Luminance was controlled by placing appropriate combinations of neutral density filters in the optical paths. Electronic shutters were used to completely block illumination. The gratings and the blank fields were mounted as slides and attached to rotary wheels locked with detents that permitted rapid and repeatable insertion of any desired stimulus combination. The luminance levels of the displays, as seen from the starting platform, were measured before and after each session by a Pritchard photomultiplier photometer and a Salford subjective match photometer. 6. Training followed general methods of operant conditioning with food as reinforcement. The daily food ration was obtained in the testing sessions, one in the morning and one in the late afternoon. The fresh beef heart used as a reward is not a complete diet for falcons. To supplement the diet, a mouse was fed to the bird after 5 days of testing and testing was resumed 2 days later. The bird was weighed daily before and after testing; the average weight was 100 g before and 110 g after a session. Weight proved to be a sensitive index of motivation as well as a sensitive indicator of general health. 7. R. D. Lord, Am. Midl. Nat. 56, 325 (1956). 8. S. Shlaer, J. Gen. Physiol. 21, 165 (1937). 9. It should be noted that any estimate of bird acuity given in cycles per degree of visual angle must be conservative, since the smaller eye of the bird will focus a smaller retinal image. Data should be related to retinal images expressed in cycles per millimeter of retina, but this requires informnation about optical characteristics of the eye, such as focal length. This information is not available for the kestrel, and in general there is a paucity of data about the optical characteristics of the eyes of Falconiformes. We thank T. Yates for an informative discussion about the problems involved in measuring the optical properties of animal eyes. 10. The thresholds for the two observers were obtained by the method of limits; monocular viewing was used for comparison with the thresholds obtained by S. Shlaer. Binocular viewing would likely have produced a significant reduction in threshold, on the basis of prior work on binocular summation; see, for example, the review by R. Blake and R. Fox, Percept. Psychophys. 14, 161 (1973). 11. We thank J. Enoch for informative discussions concerning the data required to determine the energy absorption properties of cones. 12. We thank R. Bush for his assistance in data collection, and the Tennessee Wildlife Resources Agency and the U.S. Fish and Wildlife Service for the excellent cooperation extended to us. The work was authorized by state scientific collectors permit No. 204 and federal scientific collectors permit PRT 7-01-C-Z-NV. Support was provided by NIH grant EY00931.