

By DE. PRIPP. 



THERE is yet another aspect of this subject which, though 

 seldom studied in connection with the microscope, is of more 

 significance in estimating the limits of vision than lens aperture ; 

 namely, the dioptric performance of the eye, and the capacity of 

 the retina to receive and transmit impressions of light. 



In Mr. Sorby's address, to which reference has been made in the 

 preceding pages, the " physiological part of the question " is 

 dismissed in a sentence because Mr. Sorby " does not believe 

 that the ultimate limit of distinct vision tvould he found to depend on 

 the constitution of the eijey And he seems the more confirmed in 

 this opinion by the inference which he draws from certain experi- 

 ments made by Dr. Pigott that '' the eye could distinguish with a 

 high magnifying power a much smaller interval than the physical 

 properties of light will permits Mr. Sorby further adds (in reference 

 to Dr. Pigott's estimate of ^^-qVo o ^^ ^ooVo^ of an inch as the 

 limit of visibility,) '* This (?J, however ^ is not what appears to he 

 the most important character of light in limiting the power of the 



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