874 CHROMATIC ABERRATION. [BOOK in. 



power of the instrument is so small, that such amount of chro- 

 matic aberration as does exist attracts little notice. Never- 

 theless some slight aberration may be detected by careful 

 observation. When the spectrum is observed at some dis- 

 tance off the violet end will not be seen in focus at the same 

 time as the red end. Again, if a luminous point be looked 

 at through a narrow orifice covered by a piece of violet glass, 

 which 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 / (Fig. 148) to be the plane of the mean focus of 

 A, the violet rays will be brought to a focus in the plane F", 

 and the red rays in the plane R. If the rays be supposed to 

 fall on the retina between J^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 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 con- 

 verging 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 experiment may be varied by blocking up 

 one half of the pupil with a piece of card and using the uncov- 

 ered half of the pupil to look through a piece of violet glass at 

 a white surface or a candle flame. The red strip will be seen 

 to have a blue edge. 



549. 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, espe- 

 cially when the eye is directed to a uniformly illuminated sur- 

 face. 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 ante- 

 rior 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 



