Text-book of ophthalmology . eye upwardor downward [or rather of both eyes, one going up the other down—divergence in avertical plane (see page 722).—D.] Very strong prisms can be overcome by convergence, weaker ones [of 5 A toSA]by divergence; and only very weak prisms (of 1 A or 2 A) can be overcome by verticaldeviation of the eyes. [For prisms and their numbering, see § 747.—D.] The ability to perform convergence, as determined with prisms, is also calledadduction, and the ability to perform divergence is called abduction. These terms hadInner be avoided, as they are already applied to deno
Text-book of ophthalmology . eye upwardor downward [or rather of both eyes, one going up the other down—divergence in avertical plane (see page 722).—D.] Very strong prisms can be overcome by convergence, weaker ones [of 5 A toSA]by divergence; and only very weak prisms (of 1 A or 2 A) can be overcome by verticaldeviation of the eyes. [For prisms and their numbering, see § 747.—D.] The ability to perform convergence, as determined with prisms, is also calledadduction, and the ability to perform divergence is called abduction. These terms hadInner be avoided, as they are already applied to denote the lateral excursionsto side) of the eyes (see page 719), which follow quite different laws. This is immedi-ately apparent when we remember that when side to side movements are to be made thein be abducted until the external margin of the cornea almost touches the exter-nal angle of the eye, while the outward movement of the eye in performing divergi- but a minimum one. \^h> also §§ (*)(), C>77, and 679.]. DISTURBANCES OF MOTILITY OF THE EYE 735 650. Monocular Diplopia.—A condition to be rigorously differentiatedfrom binocular diplopia is monocular diplopia. The former depends uponthe fact that though there is but one image of the object cast upon eachone of the two retinae, it is not thrown upon spots that are similarly situ->ated; but the latter is due to the formation of two images of the sameobject upon one retina. Binocular diplopia, therefore, disappears at oncewhen one eye is shut, while monocular diplopia persists, although but oneeye—that is, the one which sees double—is open. In this fact lies themost certain differential sign between the two kinds of diplopia. The causeof monocular diplopia is either an anomalous refraction of the rays oflight or the presence of a double pupillary opening. The former representsone form of irregular astigmatism (see § 790), and has its seat either inthe cornea or in the lens. It occurs particularly in subl
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