CHAP, ii.] 



SIGHT. 



509 



a red fringe, or a red centre with a blue fringe, according as the 

 image of the point looked at is thrown on one side or other of the 

 true focus. Thus supposing / (Fig. 69) to be the plane of the 



FlG. 69. DlAOBAM ILLUSTRATING CHROMATIC ABERRATION. 



hh is the dioptric surface, hv represents the blue, and 7<r the red rajs; V is the focal 

 plane of the blue, R of the red rays. 



mean focus of A, the violet rays will be brought to a focus in the 

 plane V, and the red 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 11, the con- 

 verging red rays will form a centre with the still diverging blue 

 rays forming a fringe round 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 still converging red ray lir as 

 it cuts the plane of the retina is, in ordinary vision, accompanied 

 by the diverging violet ray hv, and thus by a sort of compensation, 

 we see together even the rays which differ most in refraction. 



Entoptic Phenomena. The various media of the eye are not 

 uniformly 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 im- 

 perfections in the field of vision, especially 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 humour, the so-called muscce volitantes. 

 These are readily seen when the eye is turned towards 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 recognised 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 imper- 

 fections in the lens or its capsule, also give rise to visual images. 



