. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . he retina can berendered visible to the eye itself. Thus opacities and normal striaein the crystalline lens and those due to cataract can be seen by look-ing at a surface, such as a white cloud, through a pin-hole disc placedat the anterior focus of the eye. Another method of observing entoptiephenomena is to look at a luminous point, placed at a far distancefrom the eye, through a convex lens of high dioptric value. Amongsome of the simple entoptie phenomena which may be cited are thefollowing:
. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . he retina can berendered visible to the eye itself. Thus opacities and normal striaein the crystalline lens and those due to cataract can be seen by look-ing at a surface, such as a white cloud, through a pin-hole disc placedat the anterior focus of the eye. Another method of observing entoptiephenomena is to look at a luminous point, placed at a far distancefrom the eye, through a convex lens of high dioptric value. Amongsome of the simple entoptie phenomena which may be cited are thefollowing:— (1) On winking the eyes transverse stria; are produced due, prob- 157 li PHYSIOLOGICAL OPTICS ably, to wrinkles of the epithelial layer. If this is continued for somelength of time, as may be the ease after reading for a long time in ahorizontal position, striae which last for several hours are producedand give rise to a marked diplopia of horizontal lines. This conditionhas been called tarsal asthenopia. The striae produced by winking(after the drawing by George Bull) are shown in Fig. Fig. 75.—Entoptic Striae Produced by Winking the Eyelids. (After George Bull.) (2) The luminous spot is always limited by the shadow of theborder of the iris: the irregularities of the latter can, therefore, bestudied. The pupillary contraction is very readily seen on openingor closing the other eye. (3) Small circles with bright centers are frequently seen and these
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