ENTOPTIC PHENOMENA. 845 



tion are due to bodies behind the pupillary plane (3). It should be noted in 

 this connection that the impressions of the stimulated portions of the retina are 

 projected outward in the opposite direction. 



" 2. Purkinje's figure depends upon shadows thrown by the blood-vessels within 

 the retina upon the posterior percipient layer, the layer of rods and cones. In 

 ordinary vision they cannot be perceived. According to v. Helmholtz this is 

 probably because the sensitiveness of these shaded portions of the retina is greater 

 than that of the remainder of the retina, and their irritability is less exhausted. 

 As soon as the position of the vessel-shadow is changed, so that it is thrown to 

 one side, instead of directly backward, on places, therefore, that ordinarily do 

 not receive shadows from the vessels, the Purkinje figure at once appears. The 

 light must enter the eye as obliquely as possible. The experiment may be made 

 in several ways: (i) A small bright image of a light may be thrown upon the 

 sclera. As it moves up and down the vessel-figure moves with it. (2) Looking 

 directly upward at the sky, the depressed upper lid is blinked, so that momen- 

 tarily, in correspondence with the blinking movement, oblique rays of light 

 enter the lowest part of the pupil from above downward. (3) One may look 

 through a small opening toward the sky, and move the opening quickly to and fro, 

 so that shadows fall rapidly from both sides of the vessels on the neighboring 

 rods; or (4) one may look straight ahead in a dark room, and move a light to 

 and fro below the eye. Occasionally in this experiment the macula lutea is seen 

 like a non vascular shaded depression, appearing (on account of the inversion 

 of objects) on the inner side of the optic-nerve entrance. 



3. Recognition of the Movement of the Blood-corpuscles in the Retinal Capilla- 

 ries. -On looking (without accommodation) at a large bright surface, or at the sun 



FIG. 291. The Entoptic Shadows. 



through a dark-blue glass, brilliant points like tiny sparks are seen to move in 

 various tortuous paths over larger or smaller spaces. The movement appeared 

 to Landois to resemble most that of a Gyrinus swarm (small water-beetles) 

 on the surface of the water. The particles can often be seen to follow each other 

 in definite, outlined paths. According to some observers, the phenomenon is 

 due to the fact that the red blood-corpuscles in the capillaries outside of the 

 external nuclear layer act as small concave discs, concentrating the light falling 

 on them upon the rods of the retina. Each corpuscle must be in a suitable posi- 

 tion; if it turn over, the light-phenomenon disappears. Vierordt, who projected 

 the movement upon a screen, calculated, from its rapidity, that the velocity of the 

 blood-current in the retinal capillaries is from 0.5 to 0.75 mm. per second, and 

 this, in fact, corresponds to the direct observations of E. H. Weber and Volkmann 

 on the blood-current in other capillaries. Compression of the carotid artery 

 retards the movement, release of the vessel, as well as short expiratory pressure, 

 accelerate it. As Landois occasionally observed the points as dark spots on a light 

 ground, and as bright ones on a dark surface, the phenomenon is probably better 

 explained as a pressure-phosphene (according to 5) , from the friction of the blood- 

 corpuscles in the capillaries against the rods. 



4. The yellow spot appears also occasionally, when viewed with uniform 

 blue illumination, as a dark circle. In stronger light the position of the yellow 

 spot may be seen surrounded by a bright area, having a diameter about thrice 

 as large (Lowe's ring). 



5. Pressure-phosphenes, that is those phenomena that appear under the 

 influence of pressure on the eyeball, (a) Partial pressure on the eyeball induces 



