ILLUMINATION OF THE EYE, AND THE OPHTHALMOSCOPE. 



847 



phenomena and also that for objective light are diminished. At times the macula 

 lutea appears as a dark spot on a light ground, at other times as a light spot on 

 a dark ground, according to the direction of the current. If the current is broken, 

 the phenomena are reversed and the eye soon returns to rest. 



When the eye is directed toward a source of polarized light, Haidinger's 

 polarization brushes appear at the point of fixation. They may be seen if a bright 

 cloud is looked at through a Nicol's prism. They appear as bright, bluish spots 

 on a white ground, bounded by two similar hyperbolas; the dark bundle sepa- 

 rating them is narrowest in the center, and has a yellowish color. Blue alone 

 of the various colors of homogenous light exhibits the brushes. According 

 to v. Helmholtz the seat of the phenomenon is the yellow spot, and it depends 

 upon the fact that the yellow-colored elements of this spot are slightly birefringent, 

 and they absorb more of the rays in one place than in others. 



Finally, there should be mentioned the sensations of light produced by in- 

 ternal causes, by congestion of the retina (as from violent spells of coughing), 

 increased intraocular pressure and the like, or by congestion of the central cerebral 

 organs. Irritation of the psychooptical centers may induce distinct phantasms, 

 which Cardanus, Goethe, Johannes Miiller, Nageli.and others could in fact excite 

 in themselves . voluntarily. "Video quae volo, nee omnino semper cum volo. 

 Moventur autem perpetuo quae videntur. Itaque video lucos, animalia, orbes 

 ac quaecunque cupio" (Cardanus). In men suffering from delirium tremens 

 something similar at times takes place : they are able to call forth hallucinations 

 even in daytime, as soon as they think of certain things voluntary hallucinations. 



ILLUMINATION OF THE EYE, AND THE OPHTHALMOSCOPE. 



The light that enters the eye is partly absorbed by the black uveal 

 pigment, and partly reflected again from the eye, and always in the same 



FIG. 292. Apparatus for Illuminating the Back of the Eye B. 



direction from which it has entered. If a person place himself directly 

 before the eye of another, the head of the former, as an opaque body, 

 naturally cuts off a considerable number of rays. As no rays can fall 

 upon the eye from the direction of the head of the first person, none can be 

 reflected from the eye of the other, which, therefore, appears black to the 

 former, for the reason that he cuts off all those rays that could be re- 

 flected toward his eye. As soon, however, as it is possible to throw 

 light into the eye of the second person in the same direction in which 

 the first looks into the eye of the other, the eyeground at once appears 

 brightly illuminated. 



