236 The Mickoscope. 



In this way, with and without the bull's-eye, the lines were 

 distinctly shown from end to end of the diatom when the engraved 

 miiTor-bar was set at about 65^. At much less angle no resolution 

 was obtained. At a greater angle nothing was gained, and finally 

 the lines became less brilliant from the change in illumination. 



Putting on a hemispherical lens a magnificent showing was 

 made. The best effect was secured when the mirror-bar showed 

 about 50*^, but the lines remained visible until the angle was reduced 

 to 21'^. In this latter position, however, it was necessary to take the 

 light fi'om the upper portion of the mirror, so that the real angle 

 of illiunination was greater. With a small beam of light central to 

 the mirror, 33^ was the lowest angle giving perceptible resolution. 



The field was equally and brilliantly white throiighout, except 

 that the limiting edge of the diaphragm was bordered by a dark red 

 line shading to orange. When the object extended beyond the field, 

 one could follow it, distinct and white, fairly into this red boundary 

 line. It is, however, necessary to slightly change the fine adjust- 

 ment for the sharpest focus in passing from the center to circumfer- 

 ence of the field. The change required is very little, and when 

 made the results near the boundary are as good as at the center. 



I cannot so well describe the results with the Abbe condenser, 

 but at its best the resolution was certainly no less satisfactory. Ordi- 

 narily, however, I prefer for central light a low- angled half -inch 

 objective, except when simply wishing to find, mixed with other ma- 

 terial, stained objects like i?ac^7Z^ts tuberculosis. With the half-inch 

 as condenser, plane mirror, and the lamp as before, the results with 

 central light were equally good, but I could not perceive that the 

 objective gives greater promise in this respect than it does with 

 oblique illumination. I have never before seen Surirell i gemma so 

 well shown. But I failed just where I have heretofore failed in 

 resolving with central light (central beam on plane mirror and con- 

 denser carefully centered,) viz : No. 18 Moller's balsam test plate. 

 All before this came out beautifully. I have seen balsam mounted 

 Amphipleura brilliantly resolved without substage apparatus and 

 with the miiTor in central position, but by no means in this case with 

 central illumination. Have never seen the last three tests on MuUer's 

 plate brought out with truly central light. In my hands the new 

 objective does not do it. I have heretofore looked for the cilium of 

 Bacterium termo and looked in vain, though others have seen it with 

 presumably no better equipments. I have again failed with the new 



