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In this study we report on the effects of constant fight (CL) on the refractive development and ocular morphology of White Leghorn chicks (Cornell K-strain). Refractive state and corneal curvature were measured by IR photoretinoscopy and IR keratometry respectively. The axial lengths of the ocular components were measured by A-scan ultrasonography. We find that constant light produces significant hyperopia compared to controls in as few as 10 days (7.4 vs 4.0 D). This is apparently the result of flatter than normal corneal curvature (radius of curvature: 3.22 vs 3.08 mm) as vitreous chamber depth is significantly deeper in CL eyes than controls at that age (5.6 vs 5.1 mm). In contrast to other reports, if CL rearing is continued for longer periods the hyperopia progresses, even though vitreous chamber depth continues to increase. After 11 weeks of CL severe hyperopia was observed (18.2 vs 2.8 D). Long term CL is …