. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, Fig. 154.—Maddox Rod. Fig. 155. a piece of plane dark red gkss before one eye, so that thered light always corresponds to the eye with the red a Maddox rod (Fig. 154), white or red, may be usedfor the same purpose. This may be a series of rods (seeFig. 155) placed in a metal cell of the trial-case, and theeye, looking through it at the light, will see the image ofthe flame distorted into a streak of broken light. A strong4- cylinder from the will an
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, Fig. 154.—Maddox Rod. Fig. 155. a piece of plane dark red gkss before one eye, so that thered light always corresponds to the eye with the red a Maddox rod (Fig. 154), white or red, may be usedfor the same purpose. This may be a series of rods (seeFig. 155) placed in a metal cell of the trial-case, and theeye, looking through it at the light, will see the image ofthe flame distorted into a streak of broken light. A strong4- cylinder from the will answer the .same pur-pose. As the rod refracts rays of light opposite to its axis,the eye will see a streak of light in the reverse meridian to MUSCLES. 183. that in which its axis is placed. To expedite the deter-minations, tlie rotary prism of Cretcs or the revolvingprisms of Rislcy may be employed. This latter apparatus(see Fig. 156) is composed oftwo superimposed prisms of i 5centrads each, and mounted in amilled-edged cell of the size ofthe trial-lens. By means of amilled-edged screw these prismsare made to revolve so that inthe position of zero they neutral-ize each other, and when rotatedover each other the prismstrength gradually increases un-til the bases of the prisms come together and equal 30centrads. The strength of the prism employed is indicatedby an index on the periphery of the cell. Stevens Phorometer.—This is a very convenient appa-ratus, composed of two 4-degree prisms placed in a frame3 ^2 inches from the eyes, and with an attached lever canbe rotated so as to test the strength of the vertical andlateral muscles. Indexes and letters at the periphery ofthe frame record the character and degree of the insuffi-ciency. (
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