Some Ahhe Letters, 239 



" ^wt I observe that in all these experiments the different 

 aspects of the object will appear in the same way, if a broader 

 beam of light is applied and the spectra are superposed, pro- 

 vided that the diaphragms, i.e. the holes in them, are exactly 

 conform to the distances of corresponding points in the 

 superposed spectra." 



I think we may account for at least some of the difficulties 

 which people have had in following Abbe's theories by the fact 

 that his English was not always of the best, and his critics were 

 not always quite conversant with German. As a little illustra- 

 tion : What ought we to say to tlie amusing passage in the letter 

 this evening :— 



" Since microscopists in general have no, or little, under- 

 standing " ? 



'o 



That certainly seems a trifle hard on the poor community of 

 microscopists, but if we turn it back into German* and read it in 

 connexion with the context we find it only means that they have 

 not in general a sufficient grasp of physical and mathematical 

 optics — a complaint which I am afraid we must put up with. 



I will only refer to two more passages of interest in the letters 

 to Stephenson. One of them in which he says : — 



" The outlines of any object are delineated by the direct 

 rays alone in bright field, by diffracted rays only in dark 

 field," 



is the sort of thing which is liable to give rise to a misconception, 

 which I believe actually took place at the time, viz. : that the 

 image formation of outline and structure of an object was produced 

 by a different species of mechanism of the light-waves. What of 

 course Abbe actually refers to is the fact that in ordinary or bright 

 field illumination, as he calls it, the direct rays to the exclusion of 

 the diffracted rays, suffice for the general, if somewhat hazy, out- 

 line of an object to be visible. In dark ground (or as he calls it 

 dark field) illumination he points out that the outline of an object 

 is seen by diffracted pencils only ; that of course becomes self- 

 evident when we bear in mind that dark ground illumination only 

 arises when the direct or dioptric beam does not enter tlie 

 objective. 



The last point in the letters which I will call attention to is 

 the interesting passage saying he will be glad if Mr. Stephenson 

 showed some of his experiments, not " as paradox plienomena," 

 but as phenomena illustrating a distinct idea of the functions of 

 the microscope, and again where he emphasizes that the scientific 



[* The three letters read were written by Abbe in English. — F.J.C.] 



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