198 



NOTE. 



Tv:o Letters from Professor Abbe to John Ware Stephenson. 



Communicated by F. J. Cheshiee. 



Two Facsimile Figs. 



I send herewith for the pages of the Journal of the Eoyal Micro- 

 scopical Society copies of tAvo letters, already of historical interest, 

 relating to the design and productioa of the first Microscope 

 homogeneous-immersion objective, made with the object, inter alia, 

 of testing the possibility of obtaining enhanced resolving power by 

 the use of such an immersion liquid. These letters, which were 

 written by Professor Abbe to the English microscopist, John Ware 

 Stephenson, towards the close of the year 1877, speak for them- 

 selves. I am able to bring them to the notice of the Eoyal 

 Microscopical Society by the courtesy of their present owner. 

 The first of these letters reads as follows : — 



Jena, Nov. 14, 77. 



Dear Sir, 



I beg your kind indulgence for the unpolite silence which, Mil 

 now, I have opposed to your letters. I hope you will pardon me, con- 

 sidering that I am a much occupied man. I wished to give a distinct 

 answer to your question about oil-iramersion objectives ; and, being 

 prevented from studying the question by want of time, writing has been 

 protracted so long. 



Now the matter is, that from a different point of view, some time 

 ago I have considered already the oil-immersion. I had in view the 

 advantages of this arrangement for petrograpbic work, i.e. the observa- 

 tion of what we call " Dunnscliliffe." As those thin plates of minerals, 

 which are used for microscopic inspection, generally are roughly ground 

 and not well polished, their observation with higher powers, even with 

 immersion lenses, has some difficulty, which a fluid of higher refraction 

 would prevent. In this aim I have made a preliminary calculation 

 already in last winter ; but I had not time to finish it. Your suggestion 

 now induced me to study this object from a more general point of view, 

 especially to take in consideration the increase of aperture, which this 

 mode of immersion could possibly aflord. 



Some days ago I have succeeded in the calculation of \" on this 

 principle, which, I hope, will prove some advantage compared with the 

 common immersion lenses. I have got a formula for the highest 

 aperture, which, I think, has been realized till now, 112° Z»«/sam-angle 



