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SUMMARY OF CURRENT RESEARCHES RELATING TO 
under fairly similar conditions, to make it do what is reported for the 
others. 
The first tests were made upon Moller’s balsam-mounted test-plate, 
with an ordinary small coal-oil lamp, with flat wick 3/4 in. wide, and 
common round chimney, on which, however, was placed a tin extension 
16 in. long, to improve the combustion and steady the flame. Any one 
who sits down to a prolonged task of this kind will appreciate the latter, 
at least, of the improvements thus obtained. The lamp was placed 
30 in. from the mirror to the left, with the centre of the flame used 
edgewise, and mirror of the same height. The mirror-bar was placed 
at the angle of 51° from axis of the instrument, and the concave side 
accurately focused by means of the paper label on the test-slide. No 
substage helps of any kind were used. After adjustment of objective 
and light as described, an ordinary bull’s-eye, same height as flame or 
mirror, was pushed in and out at will near the lamp, flat surface to the 
latter. The tube of the instrument could be closed to 6^ in., measured 
from its lower end to top of draw-tube, and could be elongated to 13 in. 
The Tolies’ and Spencer’s objectives have screw collars ; these were 
adjusted for their best effect with tube-length of 10 in., measured from 
front of objective. For the others the tube was varied to suit. It 
should be said that the Zeiss objective was ordered for the long tube. 
Amphipleura pellucida, on this particular slide, is of medium grade as 
to difficulty of resolution, but as difficult as any I have seen in Moller’s 
test-slides. No. 19 is probably proportionally easier, often showing by 
light and adjustments which No. 18 defies. I think this last is unusually 
difficult, and the same may be said of No. 12, Grammatopliora subtilissima. 
The others seem to be fair average shells. As immersion media, some- 
what thickened cedar oil, as furnished by Zeiss, and a fluid sent out by 
the Gundlach Optical Company, were used successfully with all the 
objectives, with, however, no perceptible difference in result. The work 
was done in the daytime, with windows behind the operator, uncurtained. 
There were no windows in front or at the sides. 
Under these conditions all four objectives resolved Amphipleura so 
plainly that any tyro could make out the transverse lines, at least, when 
the bull’s-eye aided the illumination. Often the lines appeared the 
moment the focus was secured, and this could be changed back and 
forth with almost certainty that they would be evident whenever the 
proper adjustment was made. I need not say, however, that it always 
required careful work, and that there were failures as well as triumphs. 
The two non-adjustable objectives did best with the shortest tube and 
negative ocular. With Zeiss’s compensating ocular the result was rather 
more satisfactory with the 10-in. tube-length. There did not appear to 
be the same difference with the Gundlach in this respect, the Zeiss eye- 
piece also showing well with the short tube. With the apochromatic 
at its best, the diatom appeared perfectly flat, with midrib and margins 
showing distinct and clear, when the lines were in focus — a thing none 
of the others did, though Spencer’s, perhaps, came nearest to it at 8, 
with 10-in. tube. The whole field, too, of the first named, including 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 
