INTRA-RET1NAL IMAGES. 469 



are smaller. The infra-retinal images include those of the bloodves- 

 sels, and the blood moving in them, described by Purkinje, who 

 showed, by means of a simple experiment, that most persons are able 

 to see the shadows of their retinal bloodvessels. The vascular image 

 of the retina, or the image of Purkinje, is best perceived, by moving 

 a lighted candle up and down, or in a circle, a few inches in front, 

 and to one side, of the eye, so that the light may enter it obliquely, 

 this being done in a dark room, or when the eye is directed towards a 

 dark surface. The appearance of a bright field, moving with, and 

 before, the eye, is then gradually excited ; on it are seen dark arbor- 

 escent vessels, branches of the central artery and vein of the retina, 

 and even images of the optic eminence and yellow spot. The rods 

 and cones of the retina are excited by the luminous rays, its general 

 surface being thus illuminated ; but the parts of its bacillary layer, 

 which are covered by the vessels, being relatively more protected from 

 the light, are perceived as dark arborescent ramifications, always much 

 larger than the vessels themselves, since they are projected into the 

 visual field. In accordance with the principles of visual direction, the 

 appearances are, moreover, reversed, the optic eminence of the right 

 eye being seen on the outer side of the yellow spot. The shadows of 

 the vessels shift considerably, when the candle is moved about ; and, 

 as these vessels lie chiefly behind the nerve-fibres, but in front of the 

 rods and cones, it would certainly appear that the last-named parts 

 are the recipient portions of the retinal structures. Another, and 

 more striking, experiment consists in looking steadily at a uniform 

 field of light, such as the sky, or an illuminated ground lamp-glass, 

 and rapidly moving the finger, to and fro, in front of the eye. After 

 a time, delicate reddish images of the retinal capillaries, and even of 

 the blood corpuscles moving in them, are seen on the luminous field. 

 This form of mtra-retinal image has been made use of, to determine 

 the velocity of the blood in the capillary vessels : it appears to be from 

 yl^th to T o tn f an i ncn P er second (Vierordt). Pressure will also 

 produce entoptical images of the retinal vessels, and also of those of 

 the choroid, shining on a silvery blue ground. 



The dark field observed when the eyes are closed, or kept open in a 

 perfectly dark room, is not absolutely black ; it is often, from retinal 

 excitement, covered with an obscure luminosity, and sometimes with 

 minute points of light, like luminous dust. This field is not circular, 

 but elliptical, like the ordinary visual field; it is projected into the 

 exterior, moves with the eyes, and is itself the result of a positive sen- 

 sation, quite distinct from blindness. The size of this dark field di- 

 minishes in the horizontal direction, when the closed eyes are made 

 to converge. 



The curious spectra, named phosphenes, caused by pressure, with 

 the finger, on the closed eye, have been previously mentioned, as well 

 as the fact, that these luminous impressions are always referred to a 

 direction opposite to that of the seat of pressure; pressure on the in- 

 ner side of the eyeball, for example, excites a luminous spectrum which 

 appears on the outer side. It was noticed by Muller, that pressure, 

 with the finger, upon the eye, gives rise to a variety of luminous irn- 



