Page:Popular Science Monthly Volume 9.djvu/714

686 One of the most interesting entoptic phenomena is the Arborescent Figure, discovered by Purkinje. If, toward evening, we place ourselves 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 beyond, 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



the retina at b. At this particular spot no vessels will be seen, because 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 movement as the light. We do not, however, generally perceive these retinal vessels, because usually the light falls upon the retina from all