ENTOPTICAL PHENOMENA. 763 



figure of the blood-vessels moves in the same direction. (2) Look directly upwards to the sky, 

 wink with the upper eyelid drooping, so that for a moment, corresponding to the act of winking, 

 rays of light enter obliquely the lowest part of the pupils. (3) Look through a small aperture 

 towards a bright sky, and move the aperture rapidly to and fro, so that from both sides of the 

 blood-vessels shadows fall rapidly upon the nearest series of rods and cones. (4) In a darkened 

 room look straight ahead, and move a light to and fro close under the eyes. Occasionally, 

 whilst performing this experiment, one may see the macula lutea as a non-vascular shaded de- 

 pression, and, owing to the inversion of the objects, it lies on the inner side of the entrance of 

 the optic nerve. 



3. Movements of the blood-corpuscles in the retinal capillaries. On looking, without 

 accommodating the eye, towards a large bright surface, or through a dark blue glass towards 

 the sun, we see bright spots, like points, forming longer or shorter chains, moving in tortuous 

 paths. The phenomenon is, perhaps, caused by the red blood-corpuscles (in the capillaries 

 posterior to the external granular layer) acting as small light-collecting concave discs, concen- 

 trating the light falling upon them from bright surfaces, and throwing it upon the rods of the 

 retina. Each corpuscle must be in a special position ; should it rotate, the phenomenon 

 disappears. Vierordt, who projected the movement upon a screen, calculated, from the 

 velocity of their motion, the velocity of the blood-stream in the retinal capillaries as equal to 

 0*5 to 0*75 mm. in a second, which corresponds very closely with the results obtained directly 

 in other capillaries by E. H. Weber and Volkmann ( 90, 4). "When the carotids are com- 

 pressed, the movement is slower on freeing them from the compression ; during short forced 

 expirations the movement is accelerated {Landois). 



4. The entoptical pulse ( 79, 2) depends upon the pulsating arteries irritating mechanically 

 the rods lying outside them. 



5. Pressure Phosphenes. Pressure applied to the eye causes a series of phenomena: (a) 

 Partial pressure upon the eyeball causes the so-called illuminated "pressure-picture" or 

 phosphene, which was known to Aristotle. As the impression upon the retina is referred to 

 something outside the eye, the phosphene is always perceived on the side of the field of vision 

 opposite to where the pressure, affects the retina, e.g., pressure upon the outer surface of the 

 eyeball causes the flash of light to appear on the inner side. If the retina is not well lighted, 

 the phosphene appears luminous ; if the retina is well lighted, it appears as a dark speck, 

 within which the visual perception is momentarily abolished, (b) If a uniform pressure be 

 applied to the eyeball continuously from before backwards, as Purkinje pointed out, after some 

 time there appear in the field of vision very sparkling variable figures, which perform a wonderful 

 fantastic play, and often resemble the sparkling effects obtained in a kaleidoscope (v. Helm- 

 holtz), and are probably comparable to the feeling of formication produced by pressure upon sen- 

 sory nerves ("sleeping of the limbs"), (c) By applying equable and continued pressure, 

 Steinbach and Purkinje observed a network with moving contents of a bluish-silvery colour, 

 which seemed to correspond to the retinal veins. Vierordt and Laiblin observed the branching 

 of the blood-vessels of the choroid as a red network upon a black ground, (d) According to 

 Houdin, we may detect the position of the yellow spot by pressure upon the eyeball. 



6. The entrance of the optic nerve may be detected on moving the eyes rapidly backwards, 

 and especially inwards, as a fiery ring or semicircle about the size of a pea. Probably, owing 

 to the movement of the retina, the entrance of the optic nerve is stimulated mechanically by 

 the rapid bending. Purkinje and others observed that the ring remained persistent on turning 

 the eye strongly inwards. If the retina be brightly illuminated, the ring appears dark, and 

 when the field of vision is coloured, the ring has a different tint. If Purkinje's figure be pro- 

 duced at the same time, one may observe that the vascular trunk proceeds from this ring a 

 proof that the ring corresponds to the entrance of the optic nerve (Landois). 



7. Accommodation Spot. On accommodating the eye strongly towards a white surface, 

 there appears in the middle a small, bright, trembling shimmer, and in its centre a coarse 

 brown speck, about the size of a pea, is seen (Purkinje). If pressure be applied externally to 

 the eyeball, this speck becomes more distinct. After having once observed the phenomenon, 

 occasionally on pressing laterally upon the opened eye we may see it as a bright speck in the 

 field of vision another proof that the intraocular pressure is increased during accommodation. 



8. Mechanical Optical Stimulation. On dividing the optic nerve in man, as in extirpation 

 of the eyeball, a flash of light is observed at the moment of section by the person operated on. 

 The section of the nerve-fibres themselves is painless, but section of the sheaths is painful. 



9. The accommodation phosphene is the occurrence of a fiery ring at the periphery of the 

 field of vision, seen on suddenly bringing the eyes to rest after accommodating for a long time 

 in the dark (Purkinje). The sudden tension of the zonule of Zinn resulting from the relaxation 

 causes a mechanical stretching of the outermost part of the margin of the retina, or it may be of 

 a part of the retina behind this. Purkinje observed the phenomenon after suddenly relaxing 

 the pressure on the eye. 



10. Electrical Phenomena. Electrical currents, when applied to the eye, cause a strong 

 flash of light over the whole field of vision. One pole of the battery may be placed on the 

 under eyelid and the other on the neck. The flash at closing [making] the current is strongest 



