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A "NEW" OBJECT GLASS BY ZEISS. 



By J. W. Gordon. 



{Read March 23rd, 1915.) 

 Figures 1 and 2. 



In the November number of the Journal of the Club there 

 is a paper by Mr. Nelson upon a new object glass by Zeiss. 

 Of this object glass Mr. Nelson speaks in high praise, and no 

 doubt it merits the encomium which he bestows upon it. Besides 

 describing its performance, Mr. Nelson attributes entire novelty 

 to the plan upon which this objective is constructed. That plan 

 is the fitting of an oil -immersion front lens to a |-in. dry 

 objective so that an oil-immersion objective is produced having 

 a numerical aperture less than 1. Mr. Nelson certifies that, so 

 far as his knowledge goes, this type of objective is quite new. 



From the merit of Messrs. Zeiss in recognising the advantage 

 to be secured by applying the oil -immersion front lens in this 

 way, I do not at all wish to detract ; but it is perhaps worth 

 while to point out that the idea is not quite so new as Mr. Nelson 

 supposes. So far back as July 1909, Messrs. It. &, J. Beck 

 produced and supplied to me a lens which was precisely of this 

 type, and in design identical with this new lens of Zeiss, although 

 in fact the oil-immersion front lens was applied to a g-in. dry 

 objective. This lens I have had in constant use since then, and 

 have exhibited it on various occasions. I am a little surprised 

 to learn from Mr. Nelson's paper that I have not actually shown 

 it to him. It was catalogued for exhibition at South Kensington 

 at the time of the Optical Convention, and although there was a 

 difficulty about the space, so that the lens itself was not actually 

 set up there, the following description of it appears in the 

 catalogue : 



" The use of oil immersion has hitherto been confined to 

 objectives of the l/8th-in. and l/12th-in. class under an im- 

 pression, which proves to be mistaken, that oil immersion secures 

 no particular advantages when applied to objectives of lower 

 power. The model is a |-in. dry lens fitted with a supplementary 

 lens of rather less than hemispherical angle, mounted so that the 



