854 THE SENSE OF SIGHT 



vitreous humor. In order to render the latter visible, the eye should 

 be turned upward upon a uniformly illuminated surface, when they 

 will place themselves directly in the line of vision, and give rise to a 

 sensation of beads, strings or patches floating through the visual field. 

 On account of their almost constant motion, which may be increased 

 by movements of the head or eyes, they are known as the muscce voli- 

 tantes. They are said to represent the remains of the embryonic struc- 

 ture of the vitreous humor, such as cells which have failed to undergo 

 a complete transformation into vitreous substance. These fleeting 

 visual sensations belong to the group of the entoptic phenomena, be- 

 cause they are produced by objects within the eye. 



The human eye also shows an imperfect centration of its refractive 

 media. In the horizontal meridian the optical axis of the cornea 

 differs from that of the lens by 0.3°, and in the vertical meridian by 

 as much as 1.3°. Furthermore, attention has already been called to 

 the fact that the optical axis of the eye does not coincide exactly with 

 the visual axis. Naturally, the most perfect system would be the one 

 in which the refractive media are accurately centered upon an axis 

 which strikes the retina in its most sensitive area. 



Reference has already been made to the fact that the crystalline 

 lens is not free from spherical aberration, the rays passing through 

 its peripheral zone being converged more than those traversing its 

 center. It is also open to chromatic aberration, the violet rays being 

 brought to a focus in a point closer to the lens than the red rays. Like 

 in all optical instruments, these aberrations are minimized by a stop 

 in the form of the iris which shuts out its marginal zone. In spite 

 of this fact, however, we still obtain a slight spherical aberration 

 which, together with the imperfect centration of the refracting media, 

 gives rise to a mild degree of astigmatism. Thus, a star or the light 

 of a lantern is not seen as a round luminous point, but as beset with 

 radial streamers. Quite similarly, the chromatic aberration still 

 remaining, frequently amounts to 0.5 mm. as far as the violet and red 

 rays are concerned. This condition, however, does not interfere 

 appreciably with the clearness of the retinal image, at least, not with 

 the impression produced by it in consciousness. Admittedly, the 

 retina becomes abruptly insensitive toward the rays at the extreme 

 ends of the spectrum, and is more readily excited by the rays in and 

 near the yellow. Consequently, the absence of chromatic aberration 

 in" our eye is due to the fact that the iris prevents refraction through the 

 peripheral zone of the lens, and secondly, to the physiological and not 

 to the optical qualities of our eye. 



Among these dioptric defects of our eye might also be mentioned 

 the entoptic phenomena produced by the tears anointing the anterior 

 surface of the cornea, as well as by the particles of mucus, globules 

 of fat and dust contained therein. The latter are constantly removed 

 from in front of the pupil by the movements of the eyelids. Sub- 

 jective visual impressions atso result in consequence of the heterologous 



