186 On Zeiss ^\fh Immersion. By W. J. HicJcie. 



exact adjustment of the correction-screw required for a considerable 

 number of the most trying tests. Having carefully compared these 

 glasses, one with another, and likewise with Seibert's ^V^h and 

 Schieck's (so-called) 4Vth, both immersions, I left off with a strong 

 impression that the one I have called C was markedly superior to 

 the other two, reserving the more rigid scrutiny for the following 

 morning. As my habit is, when I have any task of this kind 

 requiring more than ordinary carefulness, to work during the 

 morning hours, and in a darkened room, it gave rise in this 

 instance to a somewhat curious accident. Having arranged the 

 lenses on a sideboard, and, as I thought, in the order in which I 

 have named them, i. e. A, B, C, I renewed the examination of the 

 preceding evening, but this time with the utmost care and exact- 

 ness. On this occasion, however, to my great disgust, the decidedly 

 best glass appeared to be B; and repeated trials only made its 

 superiority more and more evident. Unwilling to believe that I 

 could have been so maladroit the night before in a matter so 

 straightforward as this, I proceeded to light a candle, and the 

 mystery was solved at once. The glass I had taken up second on 

 each occasion turned out to be G, which had somehow got placed 

 second in order instead of third. As a matter of course, I tried 

 them all again the morning after that ; but the result remained 

 the same, and C again proved itself to be the best glass all through. 

 A came next in order of merit, and then, but at a considerable 

 interval, B. It is therefore of C that I shall have to speak in this 

 present paper, as the other two passed immediately afterwards into 

 other hands. I may add, that the maker had himself compared 

 them very carefully, and had been unable, as he declared to me, to 

 detect any difference whatever. Before proceeding further, I will 

 mention briefly the most salient features of this class of objective. 

 The workmanship is conspicuous above all German objectives which 

 I have seen for its extreme neatness, and may be examined all over 

 with a hand-magnifier. There is no extravagant disparity between 

 the breadth of the front lens and the breadth of the hindermost 

 lens ; upon which point I shall have something to say later on. Its 

 working distance is remarkably great. But this is characteristic of 

 all Zeiss' objectives. It has abundance of light, even with deep 

 eye-pieces. This C now in my possession bears Ross' E eye-piece, 

 with very little ill effect beyond becoming more difficult to manipu- 

 late. It also has complete flatness of field, and is wondrously 

 sensitive to the slightest change of the correction-screw. The 

 following instance will suffice to show this. On Stauroneis spicula, 

 when placed vertical, and with the correction-screw set at 11^, the 

 transverse striae are altogether invisible ; but when the correction- 

 screw is at 12^, the same striae are brought into view with perfect 

 clearness ! The screw-collar, instead of moving the front lens, as 



