A technique for measuring changes in the refractive power of the eye in experimental animals is described. This technique makes use of near infrared beam collimated on the retina and recorded, after reflection from it, by a photocell system. The output of the photocell system is related linearly to the changes in accommodation. The effects of fluctuations in source intensity, and of electrical pickup of noise, are rendered negligible by a special arrangement of the photocells. The system offers high sensitivity and stability, without requiring mechanical or optical modulation of the infrared beam. It employs relatively inexpensive, commercially available components. The optical properties of the cat’s eye are discussed, as are also the requirements for application of this technique. The possibility of accommodation by axial movement of the lens, and measurement of such movement by the present system, are considered. A procedure for objective calibration in diopters is described. The technique has been employed in experimental work on the cat. The smallest biological signals are significantly greater than the threshold of the system, which is about 0.01 diopter.
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