846 ENTOPTIC PHENOMENA. 



the so-called luminous pressure-picture or phosphene, which was known to 

 Aristotle. By the projection of this retinal stimulation outward, the phos- 

 phene is perceived on the side of the visual field opposite to that where the pressure 

 was made upon the retina. For example, pressure on the outer side of the eye- 

 ball causes the light-phenomenon to appear on the inner side. If the retina is 

 darkened, the phosphene appears bright; if the retina is illuminated, the phos- 

 phene appears as a dark spot within which the visual sensation is momentarily 

 abolished. (6) If uniform pressure from before backward be made for some time 

 on the eyeball, there appear in the field of vision after a time, as Purkinje pointed 

 out, bright, changing figures which produce a strange phantastic play, often similar 

 to the most brilliant kaleidoscopic pictures probably comparable to the feeling 

 of formication produced by pressure upon the sensory nerves (limbs "going to 

 sleep"), (c) By applying equable and continued pressure, Steinbach and Purkinje 

 saw appear a vascular network of a bluish-silvery color, with streaming contents, 

 which seemed to correspond to the retinal veins. Vierordt and Laiblin recognized 

 in addition the ramifications of the choroidal vessels, red on a dark background, 

 as a network with the forms characteristic of these capillaries, (d) According 

 to Houdin it is possible also to recognize the position of the yellow spot by pres- 

 sure on the eyeball. 



6. The entoptic pulse-phenomenon belongs to the pressure-phosphenes, and 

 depends upon the mechanical stimulation of the optic-nerve fibers by the pulsating 

 retinal vessels. 



7. The point of entrance of the optic nerve may be perceived on rapid, jerking 

 movements of the eye, especially inward, as a fiery circle or semicircle, slightly 

 larger than a pea. Probably the retina around the nerve-entrance is irritated 

 mechanically by the bending of the nerve. Landois saw this ring, as did Purkinje, 

 remain persistent when the eye was turned strongly inward. If the retina is strongly 

 illuminated, the ring appears dark, and when the visual field is colored, the ring 

 has a different hue. If Purkinje's figure be produced at the same time, the blood- 

 vessels appear to spring from this ring a proof that the ring corresponds to the 

 optic-nerve entrance. 



8. Accommodation-spot. If the eye is accommodated as strongly as possible 

 for a white surface, a small bright, vibrating shimmer appears, in the middle 

 of which a brownish spot the size of a pea may shortly be observed. If pressure 

 be made on the eyeball at the same time, this spot becomes more distinct. When 

 this phenomenon is once recognized, a brighter spot may be seen in the middle 

 of the visual field when lateral pressure is made upon the open eye, another proof 

 that the intraocular pressure rises during accommodation. By producing the 

 preceding phenomenon (No. 7) it is demonstrated that the phenomenon takes 

 place at the optic-nerve entrance. 



9. The accommodation-phosphene consists in the appearance of a fiery ring 

 at the periphery of the visual field when the eyes are .suddenly allowed to come 

 to rest after prolonged, intense accommodation for near vision in the dark. The 

 sudden tension of the zonule of Zinn resulting from the relaxation produces a me- 

 chanical stretching of the edge of the retina, or more probably of the retina just 

 beyond. Purkinje saw the phenomenon also after sudden cessation of pressure 

 upon the eye. 



10. Mechanical Irritation of the Optic Nerve. When the optic nerve is severed 

 in man, in the course of an operation, a bright flash appears at the moment of 

 section. The incision through the optic-nerve fibers is painless ; only the sheaths 

 are sensitive. 



1 1 . Electrical Phenomena. With variations in an electric current (one pole 

 on the upper lid, the other on the neck) bright flashes of light shoot over the entire 

 visual field. The closing flash is stronger with an ascending current, the opening 

 flash stronger with a descending current. A uniform, constant, ascending cur- 

 rent applied to the closed eye reveals the optic-nerve papilla as a dark disc in a 

 whitish-violet field. At the same time sensibility for white is increased, that 

 for black diminished. With a descending current, on the other hand, the visual 

 field appears greenish-yellow, and darkened, and in its midst the position of the 

 nerve appears light blue. If external colors are observed at the same time, these 

 tones blend to form violet or yellow with the colors looked at. Under the in- 

 fluence of an ascending current external objects are said to be seen less distinctly 

 and diminished in size when the eyes are open; while a descending current makes 

 them more distinct and larger. During anelectrotonus of the retina (in con- 

 formity with the laws of electrotonus) the sensibility for the electrical light- 



