ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
259 
and margins glowed with red, shading to dark, and a little movement of 
the focus downward 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 illumination, &c., 
destroyed the resolution to a more marked degree than with the two 
others just named ; and though the lines were beautifully 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, how- 
ever, that the magnification was less, and this I could not fairly make 
up with higher eye-piecing. Under a solid 1/4-in. 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 Tolies’ objective gave the outlines readily enough, but partaking 
somewhat of the character just described, with the solid ocular. With 
the magnification reduced to that of the 1/12 by the use of longer eye- 
pieces the haziness of lines partially disappeared, but in no way seemed 
so beautifully sharp as in the other cases. In both the Spencer and the 
Tolies there was a tinge of red in the raphe, in some cases merging into 
a dark shade, when the lines showed best under 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-eye required changing. This was varied, too, during the same 
test by inserting a narrow-angled 1/5-in. dry objective as a condenser, 
taking great care that it was in central position. In each case, to be 
further assured that the illumination was axial, examination was made 
by removing the ocular and looking at the bright spot in the back lens 
of the objective. The difference in the performance of the objectives 
was certainly less marked than with oblique light. The required tube- 
lengths remained about as before stated, with, however, less noticeable 
difference in a given amount of change. I obtained a kind of a glimmer 
of resolution on No. 19 with the apochromatic and Gundlach’s lenses, 
but nothing with any of them on 18 or 20. The others were well re- 
solved by all four objectives, Gr animat ophor a subtilissima giving the most 
trouble. I have never seen a balsam-mounted Amphipleura resolved by 
truly central illumination, though others have reported it with several 
objectives. When using the condenser named, by moving it only a 
little to one side, lines could be made out, but no comparative tests of 
this kind were made. 
I had previously tried, with the help of an expert assistant, the three 
objectives in my possession in photographing violet-stained bacteria with 
central light, showing a scarcely appreciable difference, but favouring 
the Zeiss and Spencer over the Tolies, unless the increased difficulties 
with the higher power proved too much for the skill of the manipulators. 
I have now to add a word in regard to the durability of the apochro- 
matic, the want of which has been frequently questioned. After about 
two years’ use it became that this lens was in some way impaired, and 
