. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, us,or diplopia due to a paralysis of one or more of the extra-ocular muscles. For example : A patient looking at apoint of light focused on the macula (M) of the left eye(L), and the right eye being turned in toward the nose,receives the rays upon the nasal retina, and hence projectsthe rays outward to the right, giving a false image to theright side ; a prism of sufficient strength is then placedwith its base toward the temple (base out) over the righteye, so that the
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, us,or diplopia due to a paralysis of one or more of the extra-ocular muscles. For example : A patient looking at apoint of light focused on the macula (M) of the left eye(L), and the right eye being turned in toward the nose,receives the rays upon the nasal retina, and hence projectsthe rays outward to the right, giving a false image to theright side ; a prism of sufficient strength is then placedwith its base toward the temple (base out) over the righteye, so that the rays from the light may fall upon themacula (M), and the diplopia will be corrected. (See ) 28 REFRACTION AND HOW TO REFRACT. 3. To test the strength of the extra-ocular muscles : Apatient looking with both eyes at a distant point of lightis made to see one light just above another by placing a3 P. D., base down or up, before either eye, and if a 2^P. D. did not produce diplopia when similarly placed, thestrength of his vertical recti is then represented by 2j4P. D. The strength of the prism placed base in Fig. 22. if increased, would produce diplopia is the strength of theexterni ; and the strength of the prism or prisms placedbase outward which, if increased, would produce diplopiais the strength of the interni. 4. For exercise of weak muscles. (See p. 185.)Lenses.—A lens is a portion of transparent substance(usually of glass) having one or both surfaces are two kinds of lenses—spheric and cylindric. OPTICS. 29 Spheric Lenses.—Abbreviated S. or sph. Sphericlenses are so named because their curved surfaces are sec-tions of spheres. A spheric lens is one which refractsrays of light equally in all meridians or planes. Sphericlenses are of two kinds—convex and concave. A convex spheric lens is thick at the center and thinat the edge, (Figs. 23, 24, 25.) The following are synony-mous terms for a convex lens : (i) Plus ; (2) positive ; (3)collect
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