. Refraction and motility of the eye, with chapters on color blindness and the field of vision . a vertical band of light, while theother eye sees a horizontal one. If the vertical planes ofthe two eyes are exactly parallel in the position of rest, thesebands should intersect at right angles. If the verticalplanes are not parallel, the angles of intersection will notbe right angles, and one eye or both must evidently havebeen rotated wheel-fashion, either intorsion or extorsion. Double Prism Test.—The Maddox prism is composedof two prisms in a trial ring, so arranged that their bases 224 REFRA
. Refraction and motility of the eye, with chapters on color blindness and the field of vision . a vertical band of light, while theother eye sees a horizontal one. If the vertical planes ofthe two eyes are exactly parallel in the position of rest, thesebands should intersect at right angles. If the verticalplanes are not parallel, the angles of intersection will notbe right angles, and one eye or both must evidently havebeen rotated wheel-fashion, either intorsion or extorsion. Double Prism Test.—The Maddox prism is composedof two prisms in a trial ring, so arranged that their bases 224 REFRACTION AND MOTILITY OF THE EYE. touch, their line of junction passing exactly through thecentre of the ring. If this is placed before one eye, so thatthis line passes horizontally in front of the centre of thepupil, it will cause a monocular diplopia, and the horizontalline used as a test object will appear as two parallel , now, the other eye be uncovered, it sees a single linewhich, if no torsion of either eye has taken place, should bebetween and parallel to the other two Fig. 87. Convergence and Accommodation—Metre Angle.—If,in the position of rest, the optic axes are parallel, a pointof light infinitely distant will be seen singly withouteither convergence or divergence. If, now, this point beapproached to a distance of one metre, the eyes movethrough a definite angle known as a metre angle—(M. A.).Evidently if the object be two metres distant, the anglewill be only .5 M. A., while if the object is still seen singlyat half a metre, the convergence amounts to 2 M. A. Attention has before been called to the fact that thereis a normal relation between convergence and accommoda-tion. In the previous test, for instance, in fixing an objectat a metre, the patient not only converges 1 M. A., but if NORMAL MOTILITY. 225 his refraction is normal, accommodates 1 D.; at a halfmetre 2 M. A. and 2 D., etc. This intimate relationshipis also indicated by the close
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