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MICROSCOPIC ILLUMINATION. 



By Hamilton Hartridge, M.A., M.D., F.R.M.S, 



Fellow of King^s College, Cambridge. 



{Read May I3th, 1919.) 



Communicated by the Hon. Editor. 



Section I. 

 Theoretical Aspect of Microscopic Illumination. 



It lias been stated (1) that the principal difference between 

 the rival theories of microscopic vision lies in the mode of regard- 

 ing the illumination of the object : whereas the Fraunhoffer 

 theory traces the light from the object, as if the object itself 

 were self-luminous, the Abbe theory considers the light from 

 its source. 



This cannot be true, for the "source" of Abbe's theory is not 

 the source of light, except under exceptional circumstances ; but 

 usually it is a virtual source, placed at infinity and of such 

 dimensions that the trains of plane waves illuminating the 

 specimen appear to be proceeding from it. In support of this 

 view the following evidence may be advanced. 



(1) In the " Diffractions-Platte " experiments which were 

 designed to prove his theory of microscopic vision, Abbe directed 

 that a distant point source of illumination be used in conjunction 

 with the plane mirror. This is the most convenient means of 

 obtaining a beam of sensibly parallel light with the ordinary 

 microscope without special apparatus. In this case the theo- 

 retical consideration of the optical processes which precede the 

 formation of the final image clearly starts at the source ; but as 

 will be shown immediately, this is the only case in which it does 

 do so, for in the following cases the source is not the source of 

 light. 



(2) In the special apparatus which Abbe had constructed for 



