On Zeiss ^^th Immersion. By W. J. HicUie. 187 



is usually the case, moves the hindermost combination ; in which 

 arrangement there are certain advantages: it decentres the com- 

 binations much less, as is plainly apparent when this species of 

 objective is compared with objectives on the old plan ; the object 

 also remains in sight while the adjustment is being altered, and 

 there is at the same time less danger of smashing the covering 

 glass of the slide. It is also well seen how carefully it has been 

 corrected generally, and through all the divisions of the adjust- 

 ment, from 5 to 16 inclusive; and that even at 15, — and I have 

 only two slides (duplicates) that require so extravagant an adjust- 

 ment, — its definition is both clear and beautiful. Beyond that it 

 begins to show a little colour. This range, however, is amply 

 sufficient for all reasonable purposes. 



I must not omit to mention that certain mechanical alterations 

 have been introduced into his objectives of this year, which did not 

 appear in his earlier issues. The figures and division-lines con- 

 nected with the correction-screw have been enlarged in size, to suit 

 those who work by lamplight. In his earlier objectives these were 

 so minute, as to make it often a difficult task to discover what was 

 the precise adjustment one had fixed upon as best suited for the 

 particular object under examination. The part containing the 

 figuring and correction-screw has been slightly depressed, so as to 

 keep it more out of the way of one's fingers ; and the screw-collar 

 itself has been made to move more stiffly. In his older objectives 

 this part was sometimes so loose, that the proper adjustment often 

 got shifted in handling, which made the glass appear to work 

 badly. These alterations I cannot but regard as improvements, 

 the more so as they were introduced at my suggestion. 



I am here reminded of certain recent statements respecting the 

 magnification of Zeiss' objectives, which seem to require some 

 notice. Dr. Griffin* states the magnification of Zeiss' ^ inch, with 

 a B eye-piece, to be x 640, and the mRgnification of his xt inch 

 (dry), with the same eye-piece, to be x 875 ; whereas, if we turn to 

 Zeiss' catalogue, we find the magnification of the |th, with the 

 second eye-piece, set down at x 330, and that of his dryt A^h 

 set down at x 500. So, again, one writer in the ' M. M. J.' states 

 the initial magnification of Zeiss' 2V inch to be x 1250, and is 

 contradicted by another, who appeals to Zeiss' own catalogue in 

 jjroof that its initial magnification is only x 680. ' In both cases 



* ' M. M. J.,' No. Ixxxvi., p. 96. 



t I am sure so dispassionate a writer as Dr. Griffin will excuse me for remind- 

 ing him, that to compare a dry lens with an immersion lens, is not usually 

 considered a fair proceeding; and that he would have done better to have made 

 the comparison with Zeiss' No. 2 immersion ( -^l inch) : not that I expect the 

 result would have been much difierent, if the J^th had been matched against so 

 unique a glass as Powell and Lealand's new i inch, but it would have been a 

 fairer coin-se to adopt. 



