The Microscope. 321 



Spencer's perhaps came nearest to it when adjusted at 8, with 

 ten inch tube The whole field, too, of the first named, includ- 

 ing the object, was beautifully white. With the Gundlach, it 

 seems to me that the lines were as distinct and crisp as with the 

 Zeiss, and could be counted with reliability, a few at a time. 

 When these were best shown the raphe and margins glowed with 

 red, shading to dark, and a little movement of the focus down- 

 ward was necessary to render the margins most distinct. With 

 a longer tube, the lines more evidently stood above the outline. 

 With the Spencer, at its best, I found little changes of illumina- 

 tion, etc., destroyed the resolution to a more marked degree than 

 with the two others just named, and though the lines were beau- 

 tifully shown, and the outline fair at the same time, it seemed 

 to me that counting would be a much more difficult undertaking. 

 It should be remembered, however, that the magnification was 

 less, and this I could not fairl}^ make up with higher eye-piec- 

 ing. Under a solid ^ inch ocular, I was unable to make any 

 distinction in the quality of the lines. With all three objectives 

 they were like parallel ropes, with uneven and woolly outlines. 



The Tolles' objective gave the lines readily enough, but par- 

 taking somewhat of the character just described, with the solid 

 ocular. With the magnification reduced to that of the j\, by 

 the use of longer eye-pieces, the haziness of lines partially dis- 

 appeared, but in no way seemed so beautifull}'' sharp as in the 

 other cases, [n both the Spencer and the Tolles there was a 

 tinge of red in the raphe, in some cases merging into a dark 

 shade, when the lines showed best under the manipulation of 

 the screw collar. 



Upon the whole, it seems to me, the apochromatic, in this 

 special test was really in the lead, though the distinction had 

 to be carefully drawn. The results on other Diatoms on the 

 plate were similar, so far as could be determined, the rating of 

 the objectives remaining the same. 



I next tried the mirror in exactly central position, with other 

 things remaining the same, save as the height and position of 

 the lamp and bull's e3^e required changing. This was varied too 

 during the same test, by inserting a narrow angled i inch dry 

 objective as a condenser, taking great care that it was in central 

 position. In each case, to be further assured that the illumina- 

 tion was axial, examination was made by removing the ocular 



