The " Oil Itmnersion''^ of Carl Zeiss. 31 



way, the ^'6th and the yV^h were manifestly superior to the 

 Zeiss. Fortunately, I am relieved of any charge of improper 

 manipulation, as the " Zeiss " came furnished with oil specially 

 prepared for it, and needed no manipulation. There was 

 nothing to do but change the objective, and make the " oil," 

 which is abominably thin and runs almost like alcohol, stay in, 

 especially with thin covers, and the stage at all approaching the 

 vertical ; this feat, however, was successfully accomplished, 

 and I may add, that before applying the oil, the slide was 

 thoroughly cleaned, so that no remnants of the glycerine, or 

 water, used with the other objectives, should interfere ; a process 

 also necessary on again changing from the "Zeiss" to the 

 " Spencer." If I knew how to do a fair thing, I am sure it was 

 done at this trial, and I freely confess the result is not what I 

 anticipated. With the Spencer objectives, the outlines of the 

 frustule, and the lines themselves on the valves, were much 

 more sharply defined than with the "Zeiss." There was no 

 difficulty at all, with mirror alone, and ordinary sky light, in 

 resolving the Amphipleura dry, with the " Zeiss," what would 

 have been called superior resolution, if it had not been seen 

 better with the same light, without touching the mirror, with the 

 Spencer objectives. There was a smoky appearance with the 

 " Zeiss " on the dry mount, and an indistinct outline of the 

 valves, with a tendency to break down with the E eye piece, 

 which was entirely absent from the two objectives compared 

 with it. 



The lines were also seen on Amphipleura in balsam, 

 by day light, but requiring much stronger light to see them 

 best with the " Zeiss," indeed, closer vicinity to the sun than 

 with the " Spencer," the latter exhibited here also the lines 

 considerably sharper. With lamplight and mirror, the results 

 were the same, all the objectives resolved the balsam mounts, 

 but the difference was unmistakably in favor of the ^V^h and 

 the 3^th. With the Wenham reflex, tried many times on the 

 balsam mounted Ajnphipleura, the " Zeiss " did its best work, 

 and more nearly equalled the yV^h than it did the ^'^th; but still 

 here, the American objectives not only showed the markings 

 blacker and finer, but stood the test of deepest eye pieces 

 without flinching, better than the "Zeiss." In all these trials 

 other witnesses were present, and the difference was clearly 

 recognizable — markedly so with ordinary day light and mirror. 



