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11. — A Furtlwr Inquiry into the Limits of Microscopic Visio)i and 

 the delusive application of Fraunhofer's Optical Law of Vision. 



No. IL 

 By Dr. Koyston-Pigott, M.A., F.E.S., &c. 



(Bead 13th November, 1878.) 



Plate III. 



The writer has been more particularly led into the present subject 

 by the wide-spread belief that the limit of microscopic vision has 

 been reached by the resolution of Nobert's lines drawn at the rate of 

 112,000 per inch, which probably gives the l-2U0,000th for the 

 diameter of the smallest line supposed to be visible. It will not be 

 uninteresting to relate the history of this belief. 



The celebrated Fraunhofer (as stated in his Memoir to the 

 Bavarian Academy of Sciences, June 14, 1823) succeeded in ruling 

 lines as close as 30,000 to the Paris inch, which he found totally 

 invisible wath the Microscope. He also announced that if A, be the 

 wave-length, and the light fell perpendicularly to the surface of 

 the ruled glass, sine 6'"^ would become imaginary, and therefore the 

 lines would produce no coloured spectra ; and he concluded, says 

 Sir John Herschel, " that an object of less linear magnitude than \ 

 can in consequence never be discerned by Microscopes as consisting 

 of parts." * 



The skilful optician Nobert, believing in this result obtained by 

 Fraunhofer, utterly despaired that anyone would ever succeed in 

 descrying his finest lines on glass.f 



Now with regard to this very conclusion of Fraunhofer, Sir 

 John Herschel regards it as " one which would put a natural limit 

 to the magnifying power of Microscopes, but " which," says he, " we 

 cannot regard as following from the premises " (sic), t 



Well, Dr. Woodward first achieved the honour of resolving 

 these lines with a Powell and Lealand tV immersion in 1869 ; § 

 and in consequence of the grave doubts expressed by their maker, 

 he wrote to Dr. Barnard, a distinguished mathematician (Pres. 

 Columbia College), who replied that "with an objective that takes 



* Art. " Light," ' Eucyc. Met.,' p. 490. 



+ Nobert thus wrote to Dr. Colonel Woodward, U.S., dated Barth, Feb. 26, 

 1869. He expressed his belief that the resolution of the higlier bands is an 

 impossibility when light is permitted to fall on closely ruled lines. '• The 



formula," says he, " sin. x — j (Fraunhofer's), if by \ we designate tlie length 



of the undulations, by b the distance between two lines of the gr.ifing, and by x 

 the aTigle of the refracted rays, gives for sin. x an impossible value when 6 becomes 

 less than A;" that is to say, wIjcu the distance between two lines is less than a 

 wave-length, the lines will bei-omo invisible. 



X 'Ency.-. Met.,' art " Light," p. 490. 



§ See 'Month. Mic. Jour.,' Dec. 1869, quoted by Dr. Woodward. 



