1 58 Obituary. 



Mikroskopische Anatomie." * This paper has been excellently 

 translated into English by Dr. H. E. Fripp.f 



Startling in the simplicity with which it admitted of experi- 

 mental verification as to the resolving power of the Microscope, | 

 and other problems imperfectly understood at the time, it created 

 a considerable sensation. It also attracted to itself a good deal 

 of criticism, which Abbe dealt with in 1880 in a controversial 

 paper entitled, '' On the Limitations of Geometrical Optics, with 

 remarks on Dr. E. Altmann's paper on the 'Theory of Image 

 Formation.' " § 



About the same period there was a great controversy going 

 on in this country on the "Aperture Question," || and it is to 

 Abbe that we owe the familiar term and significance of " Numerical 

 Aperture." In a paper read before our Society, in 1877, he gave 

 a description of his well-known Apertometer, for measuring the 

 N.A. of objectives. 



One of the direct results of Abbe's diffraction theory and his 

 work on Numerical Aperture was the introduction of the Homo- 

 geneous Immersion System for microscopic objectives. Abbe, in 

 a paper in 1879 before this Society ,1f tells how he had thought of 

 realising this principle, but did not see the wide range of its use- 

 fulness till its complete advantages were pointed out to him by our 

 late Fellow, Mr. John Ware Stephenson, who had discovered the 

 principle independently.** He thereupon made the calculations for 

 a series of objectives, which were executed by Carl Zeiss, and first 

 introduced in 1878. 



Two achievements of Abbe, in their direct and indirect results, 

 influenced the history of the Microscope more than any others. 

 They stand pre-eminent. The first, to which we have briefly re- 



* Vol. ix. pp. 413-68. 



t Proceedings of the Bristol Naturalists' Society, i. (1S75) pp. 200-68. A very 

 short but lucid abstract of this paper appears in uur Journal (then the Monthly 

 Microscopic Journal) xii. pp. 30-31. which is likewise of interest, as it is the first 

 notice of the Abbe Illuminator (the two-lens' form). 



X This, and a number of other problems dealt with in Abbe'a papers, had been 

 investigated by Helmholtz at almost the same time. Though working on somewhat 

 differeut lines, both investigators arrived at closely similar results. Helmholtz — who 

 did not know of Abbe's work till his own paper, entitled " The Theoretical Limits nf 

 Optical Capacity of the Microscope," was ready for publication — acknowledges Abbe's 

 priority in a postscript. Helmholtz's paper appeared in 1874, in the Jubelband of 

 PoggendorfFs Anualen, and has likewise been translated into English by Dr. H. E 

 Fripp, Proc. Bristol. Nat. Soc.,i. (1875) pp. 413-40. 



§ Sitz. der Jen. Gesell. f. Med. und Naturwiss. (1880) pp. 71-109. 



|1 For a concise historical review, showing the part played by Abbe and others, 

 see paper by our ex-President, E. M. Nelson, on " Microscopic Vision." in the Proo. 

 Brit. Nat. Soc, viii. (1897) part ii. 



^ "On Stephenson's System of Homogeneous Immersion for Microscope Objec- 

 tives," ii. (1879) pp. 256-65. 



** For previous anticipations of the Homogeneous Immersion System, see 

 " Carpenter on the Microscope," 8th edition, p. 364. 



