194 RECENT PROGRESS OF THE THEORY OF VISION. 



rays which we see around stars and other distant lights are 

 images of the radiated structure of our lens ; and the univer- 

 sality of this optical defect is proved by any figure with diverg- 

 ing 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 threefold. 



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 

 F -- 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 in- 

 disputable 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, perfectly 

 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 l through the cornea and crystalline lens, 

 they appear of a dingy white, less transparent than the ' aqueous 

 humour ' which lies between them. This defect is most apparent 

 in the blue and violet rays of the solar spectrum : for there 

 comes in the phenomenon of fluorescence 2 to increase it. 



1 E.g. from a lamp, concentrated by a bull's-eye condenser. 



2 This term is given to the property which certain substam 



of 



