. Refraction and motility of the eye, with chapters on color blindness and the field of vision . -called peripheral scolomata, if they are distant from thepoint of fixation may never have attracted the patientsnotice. The mapping of central scotomata is often verydifficult because the eye being blind so far as its macularfibres are concerned, cannot see the fixation mark on theperimetric arc and is unable to keep any definite position. In these cases the stereoscope may be used with a setof the cards devised by Haitz for the purpose. The knownstrength of the lenses and the distance of the card
. Refraction and motility of the eye, with chapters on color blindness and the field of vision . -called peripheral scolomata, if they are distant from thepoint of fixation may never have attracted the patientsnotice. The mapping of central scotomata is often verydifficult because the eye being blind so far as its macularfibres are concerned, cannot see the fixation mark on theperimetric arc and is unable to keep any definite position. In these cases the stereoscope may be used with a setof the cards devised by Haitz for the purpose. The knownstrength of the lenses and the distance of the cards makesit possible to have their scale correspond in degrees to thatof the perimeter or tangent curtain. Each half of a cardcontains a heavily marked circle or cross which can befused even when one eye has a central defect. With thepoor eye thus fixed it is possible with the aid of very small 342 REFRACTION AND MOTILITY OF THE EYE. test objects to map out and chart many small centralscotomata which could otherwise only be suspected. Thismethod also offers a very convenient way of mapping out. Fig. 112. the normal blind spots whose average size is indicated onone of the cards. The patient should wear his distanceglasses. This idea has been elaborated by Bausch andLomb in their wide angle stereoscope (Fig. 112). THE FIELD OF VISION. 343 We secure the same end when we have the patient fixthe white centre of the tangent curtain with both eyesopen, but with a red glass before the fellow eye. If a greentest object is used it is plainly visible to the uncovered eyebut not to its fellow and if a scotoma, is present it disap-pears or changes color just as though the other eye wasoccluded. If a green glass is used the test object shouldbe red. A positive scotoma is one which the patient can see asa spot or shadow. It is generally caused by opacities in thecornea, lens or vitreous, which coming between a healthyretina and the light, cast a shadow and so are perceivedobjectively. A negativ
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