CHAP, in.] SIGHT. 1189 



while shutting out the yellow and green allows the red and blue 

 rays to pass through, there will be seen alternately an image 

 having a blue centre with 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 f (Fig. 143) 

 to be the plane of the 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 f, 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 

 f and R, the converging 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 f, the two kinds of rays will be mixed together ; 

 as will be seen from the figure, the circumferential still converging 

 red ray hr 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, though not in absolutely proper 

 focus, even the rays which differ most in refraction. The experi- 

 ment may be varied by blocking up one half of the pupil with a 

 piece of card and using the uncovered half of the pupil to look 

 through a piece of red glass at a white surface or a candle flame. 

 The red strip will be seen to have a blue edge. 



736. 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 imperfections 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. 



Tears on the cornea, or temporary unevenness of the anterior 

 surface of the cornea after the eyelid has been pressed on it, may 

 give rise to retinal images and so to visual sensations ; but in these 

 cases the cause lies outside the eye and the result can hardly be 

 spoken of as entoptic. 



Changes in the margin of the pupil appear in the shadow 

 of the iris which bounds the field of vision. If we look at a 

 bright object or luminous surface through a pin-hole in a card 

 placed close in front of the eye (in order to get the best image 

 on the retina, the pin-hole should occupy the position of the 

 principal anterior focus), the dark circle which bounds the field of 

 vision is the image caused by the shadow of the margin not as 

 might at first be supposed of the pin-hole but of the iris. This 

 is at once shewn by the changes which it can be made to 

 undergo, while the pin-hole remains motionless, by alternately 

 closing and opening the other eye ; the field of vision of the eye 

 which is looking through the pin-hole may be observed to con- 

 tract when light enters, and to expand when the light is shut off 



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