PROPERTIES OF THE RETINA. 363 



the same, and differ from one another only in intensity, the qualitative differ- 

 ences that exist among sensations being referred to a difference in reaction 

 in the end-organ in the brain, 



Entoptic Phenomena. — Under the term entoptic phenomena 

 is included a number of visual sensations due to the shadows of 

 various objects within the eyeball itself. Ordinarily these shadows 

 are imperceptible, owing to the diffuse illumination of the interior 

 of the eye through the relatively wide opening of the pupil. By 

 means of various devices the illumination of the eye may be so 

 controlled as to make these shadows more distinct and thus bring 

 the retinal images into consciousness. Some of these eutopic ap- 

 pearances are described briefly, but for a detailed description the 

 reader is referred to the classical work of Helmholtz.* 



The Blood-corpuscles. — The entoptic images that are most easily 

 recognized perhaps are those of the moving corpuscles in the capil- 

 laries of the retina. If one looks off into the blue sky he will have 

 no difficulty in recognizing a number of minute clear and dark specks 

 that move in front of the eye in definite paths. The character of 

 the movement leaves no doubt that these sensations are due to the 

 shadows of the blood-corpuscles. In fact, the shadows often show 

 a rhythmic acceleration in velocity synchronous with the heart- 

 beats, a pulse movement. By projecting the moving images upon 

 a screen at a known distance from the eye the velocity of the capil- 

 lary circulation has been estimated in man. 



The Retinal Blood-vessels. — The blood-vessels of the retina lie 

 in front of the rods and cones and must necessarily throw their 

 shadows upon these sensitive end-organs. The shadows may be 

 made more distinct and a visual picture of the vessels obtained by 

 a number of methods. For instance, if a card with a pin hole 

 through it is moved slowly in front of the eye the images of the 

 blood-vessels stand out in the field of vision with more or less 

 distinctness. The card should be given a circular movement. If it 

 is kept in one position the images quickly disappear, since the 

 retina apparently fatigues very quickly for such faint impressions. 

 A more impressive picture may be obtained by the method of 

 Purkinje. In a dark room one holds a candle toward the side of the 

 head in such a position as to give the sensation of a glare in the 

 corresponding eye. If the eye is directed toward the opposite 

 side of the room and the candle is kept in continual circular 

 movement the blood-vessels appear in the field of vision magni- 

 fied in proportion to the distance of projection; the picture makes 

 the impression of a thicket of interlacing branches. In this ex- 

 periment the light from the candle strikes the nasal side of the 



* Helmholtz, "Handbuch der physiologischen Optik," second edition. 

 I. 184. 



