VISUAL SENSATIONS. 759 



rays in the plane R. If the rays be supposed to fall on the retina between 

 V and/, the diverging or blue rays will form a centre surrounded by the 

 still converging red rays; whereas if the rays fall on the retina between/ 

 and R, the converging red rays will form a centre with the still diverging 

 blue rays forming a fringe around them. If the rays fall on the retina at 

 /, the two kinds of rays will be mixed together; as will be seen from the 

 figure, the circumferential converging red ray h r as it cuts the plane of 

 the retina is, in ordinary vision, accompanied by the diverging violet ray 

 h v, and thus by a sort of compensation, we see together even the rays which 

 differ most in refraction. 



644. Entoptic phenomena. The various media of the eye are not uni- 

 formly transparent ; the rays of light in passing through them undergo local 

 absorption and refraction, and thus various shadows are thrown on the retina, 

 of which we become conscious as imperfections in the field of vision, espe- 

 cially when the eye is directed to a uniformly illuminated surface. These 

 are spoken of as entoptic phenomena, and are very varied, many forms 

 having been described. 



The most common are those caused by the presence of floating bodies in 

 the vitreous humor, the so-called muscce volitantes. These are readily seen 

 when the eye is turned toward a uniform surface, and are frequently very 

 troublesome in looking through a microscope. They are especially obvious 

 when divergent rays fall upon the eye. They assume the form of rows and 

 groups of beads, of single beads, of streaks, patches, and granules, and may 

 be recognized by their almost continual movement, especially when the head 

 or eye is moved up and down. When an attempt is made to fix the vision 

 upon them, they immediately float away. Tears on the cornea, temporary 

 unevenness on the anterior surface of the cornea after the eyelid has been 

 pressed on it, and imperfections in the lens or its capsule, also give rise to 

 visual images. Not unfrequeutiy a radiate figure corresponding to the ar- 

 rangement of the fibres of the lens makes its appearance. 



Imperfections in the margin of the pupil appear in the shadow of the iris 

 which bounds the field of vision ; and the movements of the iris in one eye 

 may be rendered visible by looking at a bright point or luminous surface 

 through a pin-hole in a card placed close in front of the eye, in the 

 anterior focus in fact, and then alternately closing and opening the other 

 eye; the field of the first may be observed to contract when light enters and 

 to expand when the light is shut off from the second. The media of the eye 

 are fluorescent : a condition which favors the perception of the ultra-violet 

 rays. If a white sheet or white cloud be looked at in daylight through a 

 Nicol's prism, a somewhat bright double cone or double tuft, with the apices 

 touching, of a faint blue color, is seen in the centre of the field of vision, 

 crossed by a similar double cone of a somewhat darker yellow color. These 

 are spoken of as Haidinger's brushes ; they rotate as the prism is rotated, 

 and are supposed to be due to the unequal'absorption of the polarized light 

 in the yellow spot. The prism must be frequently rotated, as when the 

 prism remains at rest the phenomena fade. Lastly, the optical arrange- 

 ments have a further imperfection in that the dioptric surfaces are not truly 

 centred on the optic axis. 



VISUAL SENSATIONS. 



645. Light falling on the retina excites sensory impulses, and these 

 passing up the optic nerve to certain parts of the brain, produce changes in 

 certain cerebral structures, and thus give rise to what we call a sensation. In 

 a sensation we ought to be able to distinguish between the events through 



