232 



Fraunhofer Applies his own Diffraction Theorem to the 

 Microscope. 



A Note by Edward M. Nelson, F.R.M.S. 



The following very important quotation with regard to the 

 above subject will be found in the article "Light," by Sir John 

 Herschel, Bart., in " The Encyclopaedia Metropolitana " (1845), 

 Vol. ii. (Mixed Sciences), p. 490, art. 758 :— 



After a description of the means employed by Fraunhofer to 

 measure the angular divergence of diffraction spectra, there 

 follows a discussion of his well-known law, derived from those 

 measurements, where it is shown that, when the elements of 

 which a grating is composed are at a distance less than one 

 wave length from one another, sin 6 becomes greater than 

 unity, an impossible quantity, so that when the medium is air 

 and the pencil is direct, i.e., perpendicular to the plane of the 

 grating, no spectrum can be given off. 



Sir John Herschel then says : " Mr. Fraunhofer seems 

 inclined to conclude further, that an object of less linear 

 magnitude than X can, in consequence, never be discerned by 

 microscopes as consisting of parts, a conclusion which would 

 put a natural limit to the magnifying power of microscopes, 

 but which we cannot regard as following from the premises." 



From this passage I judge that Fraunhofer had discovered 

 that the admission of spectra of the first order within the 

 aperture of a microscope was essential for the visibility of 

 resolvable detail. 



