convex or concave lenses alone. After perusing sevei'al leading 

 medical text-books on the eye, I have found in all of them that the 

 eft'ect of prisms has been only treated of in connection with the 

 cure of structural defects of the eye. In one work only is a method 

 of calculating the necessary prismatic angle pointed out ; but owing 

 to the approximations used and the treatment of the matter in a 

 more or less arbitrary manner, and also owing to the fact that the 

 niattei lias Ixtn treited solely fiom Aduiual standpoint, there is 

 good reison to doubt ^^hethel oculists would mike aiiy piactical 

 use of the nuthod In no t(xt book do I hnd thxt the pi actual 

 usesfoi uormil sight refen ed to ilxne lia\e betii biouLtht foiwaid 

 orad\otited , much less aioaii}- tables mstrttd ^i^ing tJie u suits 





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3;= .w..,.. 



l...cta.„10,„cb.. 



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11 ,"" 



■^ ^ minimum 15 inches 



^.nctal„,5.oh 



11 :: 

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oi^m 



i"l 



II ,'''''' 





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0207.51 





l£l. 





2* mch 



Z™ 



1 



Uinimum.COiiuhe. 



