338 SUMMARY OF CURRENT RESEARCHES RELATING TO 



not ])e agreeable to have this matter spoken of long before it is a 

 matter of fact)." 



This letter is very interesting, because it shows that at the time in 

 question, so far as the comparatively small quantities of special glasses 

 required for the production of Microscope objectives was concerned, the 

 laboratory output was sufficient to enable the work to be done. This 

 fact at once points to the possibility of meeting the demand at the 

 present time for very special glasses required in small quantities only, 

 as, for example, the production of Microscope objectives by laboratory 

 rather than by factory methods. 



The production of glass on a manufacturing scale was commenced 

 at Jena in 1884, and was brought to a successful conclusion in 188G, 

 when the first catalogue of the Jena glasses was issued. 



The third letter written by Abbe is dated March 4, 1886, and was 

 accompanied by one of the first — if not the first — homogeneous immer- 

 sion apochromatic Microscope objectives made. The letter reads as 

 follows : — 



"This is a homog. immersion of 1*40 apert. and 3*0 mm. focal- 

 length, constructed by means of new kinds of optical glass which 

 have been produced on the base of a systematical research into the 

 optical ((ualities of the various elements admitting of vitrification. 

 This research has been conducted through about three years in the 

 way of laboratory work, chemical and optical, by myself and a fellow- 

 labourer of the chemical and technical line (Dr. Schott) with the con- 

 tinuous assistance of two younger scholars, chemists and physicists ; 

 and has afterwards — nearly two years ago— induced the foundation — at 

 Jena — of a technical establishment for the regular fabrication of all 

 kinds of optical glass for general use. This glass-manufactory (which 

 has been set up in 1884 by Dr. Schott, Messrs. Zeiss, and myself, with 

 the aid of a subsidy of the Prussian Government) has taken up, and 

 continued, the former experiments on the scale of fabricatory work, in 

 order to make the results available for the various branches of practical 

 optics. This is going on still — some tasks being settled (the production 

 of the silicious glasses, which is in a regular fabrication since last 

 summertime), other tasks being brought near to the aim. In the 

 meanwhile, I have gone to work with theoretical research and computa- 

 tion ; in order to find the proper formulas for the utilization of the new 

 kinds of glass in the construction of telescope objectives and Microscope 

 objectives. 



" Regarding the latter aim, a series of objectives adjusted for the 

 short continental tube is nearly finished ; another series for your 

 English Microscopes — which requires different formulas — has been begun ; 

 and you and Mr. have at hand the first specimens of that series. 



" The optical features of the new constructions, which are repre- 

 sented by this l/8th of 1'4 ap., may be defined in that way; the 

 various corrections are of a higher order than could be obtained formerly 

 (or, more strictly spoken, the residuals of the various corrections, i.e. 

 the defects of collection of the rays, are of a higher order according to 

 mathematical terminology). 1. With the old kinds of crown and flint 

 glass two different colours onlv could be collected to one focus, a 



