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THE POPULAR SCIENCE MONTHLY 



One of the most interesting entoptic phenomena is the Arborescent 

 Figure, discovered by Purkinje. If, toward evening, we place our- 

 selves opposite a dark wall in a dark room, and move a lighted candle 

 to and fro before our eyes, looking, however, fixedly at the wall be- 

 yond, we shall then, after a little practice, see this arborescent figure, 

 whose intersecting branches cover the whole of the dark space, and 

 which is unmistakably caused by the blood-vessels in the interior 

 of the eye. The field of vision assumes a reddish appearance, upon 

 which the veins stand out in dark shadows. The trunk of the figure 

 rises a little on one side of the centre, where the optic nerve enters 

 the eye, and thence branches out after the manner of blood-vessels, 

 which is undoubtedly a proof that in this experiment we see the 

 blood-vessels of the retina itself. One spot alone is free from vessels : 

 the yellow spot, which is the most sensitive to light of all parts of 

 the retina. If, now, the candle is moved to and fro, the figure will 

 also move and follow the direction of the light. 



All these observations lead to the conclusion that we are thus 

 enabled to perceive the shadows of the vessels of the retina. That 

 these vessels cast a shadow behind them is clear, but that the shadow 

 should be sufficient to cause a perception leads to the very important 

 and interesting fact that the elements of the retina which receive the 

 impression of light must lie behind the blood-vessels. The diagram 

 in Fig. 2 will explain how the shadow of a vessel can produce an 

 image. If the light is placed at a its image will be depicted upon 



Fig. 2. 



the retina at b. At this particular spot no vessels will be seen, be- 

 cause the light is too dazzling. But the image at b forms another 

 source of light, and, if there is a vessel at v, then its shadow will be 

 thrown upon c. Now, the retina projects the image perceived at c, 

 outward, through the optical centre k, to d, where the vessel appears 

 in the field of vision. If the light is now moved from a to a', then 

 the image will move from b to b', the shadow from c to c', and the 

 image of the vessel from d to d', thus performing the same move- 

 ment as the light. We do not, however, generally perceive these 

 retinal vessels, because usually the light falls upon the retina from all 



