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Corneal power and underwater accommodation in great cormorants (Phalacrocorax carbo sinensis)

Gadi Katzir^1,* and Howard C. Howland²

¹ Department of Biology, University of Haifa, Oranim, Tivon 36006, Israel
² Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14850, USA

View larger version (11K): [in a new window]   Fig. 1. Scheme of the optics of the infrared (IR) photoretinoscope used in this study (based on Schaeffel et al., 1987). L, light source; S, black metal shield covering the lower half of the camera's lens; E, eccentricity (the distance of the light source from the upper margin of the black shield); I, the highest ray above the optical axis; A, the distance from the eye to the camera. Here, in a myopic eye (focal plane at B), a real image of the light source is created at the image plane, in front of the retina, and a blurred spot appears on the retina. Reflected light entering the pupil from the back will refocus in the focal plane of the eye (B) in front of the camera and subsequently diverge. Due to the shield, only rays emerging from the bottom part of the pupil will be detected by the unvignetted part of the aperture.

View larger version (162K): [in a new window]   Fig. 2. Photokeratometry. The light points are the reflection off a cormorant's cornea of the eight light sources in the photokeratometer ring. The distance between opposite points of reflection is a measure of the corneal radius of curvature. Scale bar, 6 mm.

View larger version (81K): [in a new window]   Fig. 3. (A) A cormorant holding a fish in its bill. The reflected infrared light crescent at the bottom of the pupil indicates a state of myopia. (B,C) States of myopia (light crescent at the bottom of the pupil) observed underwater during capture attempts by two individuals.