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XVI. — On some New Objectives and Eye-pieces by R. Winkel, 



of Gottingen. 



By Edward M. Nelson. 



(Read March 15, 1911.) 



Lately, a packet of objectives and eye-pieces by "Winkel, of 

 Gottingen, was sent to me by Messrs. Angus and Co. for examina- 

 tion, and as some points that are new to " microscopy " were founfl, 

 a description may be of interest. 



First, Winkel divides both his object-glasses and eye-pieces into 

 three groups ; thus his objectives are achromatic, fluorite, and 

 apochromatic, and his eye-pieces Huyghenian, complanatic, and 

 compensating.* 



The achromatic and fluorite objectives are of much interest, as 

 they are not more or less copies of the productions of others, but 

 have an individuality distinctly their own. To explain this it will 

 be necessary to give a little history of the modern objective. Before 

 the introduction of Jena glass the outstanding secondary spectrum 

 of the old English achromat consisted of claret, or port-red, and 

 apple-green colours. This was always looked for by experts, and 

 its presence was thought to denote perfect correction. About 1870 

 (or a year or so later), Tolles, in America, altered the correction, and 

 produced some very fine object-glasses with a flaring bright red, or 

 crimson spectrum. I well remember seeing a Podura scale shown 

 with one of these glasses, a very brilliant lens, and a strong diatom 

 resolver; the exclamation marks shone out like rubies, whereas 

 if they had been viewed through an English objective of that date 

 (Lister formula) the exclamation marks would have been seen with 

 a more purple tint, something like an amethyst. 



About 1886, when Jena glass was introduced, an entirely new 

 set of phenomena appeared, pale glasses, and those which gave 

 decidedly bluish tints — which any expert of those days would have 

 unhesitatingly condemned — were found to be not only strong 

 diatom resolvers, but also to give sharp and bright pictures. For 

 a time, experts, until they had learnt the effect of the reduction of 

 the secondary spectrum by these new corrections, were all at sea, 

 and did not know where they were. 



To-day, there is in my cabinet one of these Jena glass semi- 

 apochromats, which has such a violent purple secondary spectrum 

 that it can be seen even when a peacock-green glass is used, a more 



* Winkel's Catalogue (English edition). 



