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By DE. H. FEIPP. 



SII^CE the MSS. of the foregoing translation (completed sixteen 

 months ago) of Professor Helmholtz's essay was sent to the 

 printer, I find that the investigations of Professor Helmholtz have 

 been noticed by Mr. Sorby, in his presidential address to the Eoyal 

 Microscopical Society.* This gentleman's comment on the " Limit 

 of the Powers of the Microscope " bears mainly upon the concla- 

 sions of Professor Helmholtz respecting the influence of diffraction 

 upon the image, formed in the microscope, of very fine and closely 

 ruled lines, and on the limit thereby placed to their separate 

 recognition by the eye. A formula, expressing the physical limits 

 of resolution — i.e. the measure of narrowest interspace between 

 two finely drawn lines which admits of the formation of a separate 

 and distinct image of these lines upon the retina — was deduced by 

 Helmholtz, from his mathematical demonstration of the theory of 

 diffraction, as it occurs in the microscope. Mr. Sorby, quoting this 

 formula, employs it for a few calculations, arranged in tabular 

 form, showing the limits of resolution of a series of lines, when 



* Vide Mo7ithly Microscopical Journal, number for March, 1876. 



E 2 



