. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . GICAL OPTICS thus illuminated sends diffuse light in all directions. A retinal vesselat r, for example, intercepts the rays Ar so as to form a shadow Bxon the sensitive layer of the retina. This is the shadow which is ulti-mately seen; it will be appreciated that the shadow Bx and the vessel rare actually very near together. The shadow at Bx is seen projectedin space as B2. It can also be seen from the figure that a movementof the light source toward the visual axis will cause a movement of ther
. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . GICAL OPTICS thus illuminated sends diffuse light in all directions. A retinal vesselat r, for example, intercepts the rays Ar so as to form a shadow Bxon the sensitive layer of the retina. This is the shadow which is ulti-mately seen; it will be appreciated that the shadow Bx and the vessel rare actually very near together. The shadow at Bx is seen projectedin space as B2. It can also be seen from the figure that a movementof the light source toward the visual axis will cause a movement of theretinal vessels in the same direction. Direct illumination also pro-duces these images of the vessels on the sensitive part situated behindit, but the shadow is rarely perceived under these conditions probablybecause this shadow is always formed at the same place in direct fixa-tion and the retinal layer has thus become accustomed to this as anormal procedure. Light may be concentrated on the sclera by means of a convex lens,as shown in Fig. 84, as near the sclero-corneal border as possible. The. Fig. 84.—Entoptie Observation of the Vessels by Illumination of the Sclera. dark vessels on an orange background can be seen. The vessels movein the same direction as the luminous focus as shown by their pro-jections in Fig. 84. The explanation is the same as given in connec-tion with Fig. 83; the light of the image of the flame formed on thesclera passes through this membrane and the choroid and causesshadows of retinal vessels. H. Miiller measured the distance ab ofthe displacement of the focus of light on the sclera and the displace-ment AB of the shadow of a vessel corresponding to this displacementof the light source. Necessarily the distance between the projectedimages Ax and Bx and the distance from the point Ax or Bx to thenodal point of the eye must be known in order to calculate the size ofAB. Miiller calculated that the vessel should be mm. to front of the s
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectphysiologicaloptics
