Oil Zeiss }^th Immersion. By W. J. Hickie. 189 



the better jjavt of a long sum-iier's day at Ilartnack's establish- 

 ment in Potsdam, and that particular superiority in definition was 

 just what I failed to discover ! Chacun a son gout. But it is of 

 this definition, and its supposed de^^endeuce on perfect centering, 

 that I now wish to speak. 



The popular persuasion with regard to definition would seem 

 to be, that an objective's ability to bear deep eye-piecing is a sure 

 proof of good definition ; and, vice versa, that good definition is a 

 sure proof of its ability to bear deep eye-piecing ; and, that failure 

 in these respects is a sure proof of bad centering ; and this is the 

 belief of many, of whose opinions I may not speak otherwise than 

 with respect. 



It may be briefly formulated as follows : " There can be no 

 more certain test of good definition * than that a glass bear deep 

 eye-pieces exceedingly well," 



These 1 believe to be the prevalent views with respect to the 

 intimate connection between imperfect centering and imperfect 

 definition; so that it becomes an interesting point to inquire 

 " What are the real efiects of bad centering ? " 



Let me first give an illustration, I have in my possession one 

 of Grundlach's earliest jths, brought out when he had his reputation 

 still to make, the definition of which (with a low eye-piece) has 

 always been greatly admired. Now, according to the above theory, 

 this glass ought, by virtue of its excellent definition, to bear deep 

 eye-piecing extremely well ; whereas it breaks down utterly even 

 under the light burden of a C eye-piece. Nor will the logic of 

 results be bettered, if we say, that the inability of this glass to 

 bear a C eye-piece is a plain proof that it can have no pretensions 

 whatever to fine definition ; for " facts are stubborn chiels, and 

 wiuna ding." 



If definition were possible only under the condition of abso- 

 lutely perfect centering, then definition would be something still 

 unknown to us ; for absolute perfection in this respect is a physical 

 impossibility. 



Nageli and Schwendener f have gone into this matter at some 

 length. 



They remark : " With regard to perfect centering, the highest 

 that can be attained by the optician in this direction is at best only 

 an approximation ; and it is especially in the stronger objectives 

 that he comes farther short of perfection." 



" According to common opinion, exact centering is the con- 

 dition of aplanatism ; and this is the view taken by Harting and 

 Mohl ; and yet it is contradicted both by theory and observation." 



* Dr. Branwell expresses himself (' M. M. J.,' Ixxxv., p. 43) more cautiously : 

 " Deep eye-piecing is, above; all, the surest test of correction." 

 t ' Das Mikroskop,' p. 69. 



