THE EYE AS AN OPTICAL INSTRUMENT. 219 



st ruction of the crystalline lens, the fibres of which are 

 arranged around six diverging axes (shown in Fig. 31). So 

 that the rays which we see around stars and other distant 

 liphts are imaees of the radiated structure of our lens ; 

 and the universality of this optical defect is proved by any 

 figure with diverging rays being called ' star-shaped.' It 

 is from the same cause that the moon, while her crescent 

 is still narrow, appears to many persons double or three- 

 fold. 



Now it is not too much to say that if an optician 

 wanted to sell me an instrument which had all these 

 defects, I should think myself quite justified in blaming 

 his carelessness in the strongest terms, and giving him 

 back his instrument. Of course, I shall not do this with 

 my eyes, and shall be only too glad to keep them as long 

 as I can — defects and all. Still, the fact that, however 

 bad they may be, I can get no others, does not at all 

 diminish their defects, so long as I maintain the narrow 

 but indisputable position of a critic on purely optical 

 grounds. 



We have, however, not yet done with the list of the 

 defects of the eye. 



We expect that the optician will use good, clear, per- 

 fectly transparent glass for his lenses. If it is not so, 

 a bright halo will appear around each illuminated surface 

 in the image: what should be black looks grey, what 

 should be white is dull. But this is just what occurs 

 in the image our eyes give us of the outer world. The 

 obscurity of dark objects when seen near very bright ones 

 depends essentially on this defect; and if we throw a 

 strong light ^ through the cornea and crystalline lens, 

 they appear of a dingy white, less transparent than the 

 ' aqueous humour ' which lies between them. This defect 

 * Eg. from a lamp, concentrated by a bull's-eye condenser. 



