. A text-book of physiology : for medical students and physicians . ame meridian. Bythe character of the reflected spot and the points of reversal it is possiblewith the retinoscope to determine the principal meridians, and the differencein refraction between them, that is, the degree and the axis of the astigmatism.The Ophthalmometer.—The ophthalmometer is an instrument formeasuring the curvature of the refracting surfaces of the eye. As actuallyapplied in practise it is arranged especially for measuring the curvaturesof the cornea along its different meridians. The point for which the instru
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. A text-book of physiology : for medical students and physicians . ame meridian. Bythe character of the reflected spot and the points of reversal it is possiblewith the retinoscope to determine the principal meridians, and the differencein refraction between them, that is, the degree and the axis of the astigmatism.The Ophthalmometer.—The ophthalmometer is an instrument formeasuring the curvature of the refracting surfaces of the eye. As actuallyapplied in practise it is arranged especially for measuring the curvaturesof the cornea along its different meridians. The point for which the instru-ment is designed is to obtain the size-of the image reflected from the convexsurface of the cornea. Any luminous object placed in front of the eye will givea reflected image from the cornea as from the surface of a convex mirror.If the size of the object and its distance from the cornea are known and thesize of the corneal image is determined, then the radius of curvature of the 2 vicornea is given by the equation r = _ a-, in which prepresents the distance of.