Microscopical Researches in High Power Definition. 271 



Zeiss oil lens used as condenser. Objective Wray l-10th immersion 

 of extraordinary precision of definition ; same object as before. 



Result. — Whenever the sunshine was dimmed with clonds both 

 spaces between the spines and the spines themselves are thoroughly 

 resolved into molecules at the same instant. (A very difficult 

 operation.) A slotted U-stop replacing the small diaphragm aperture 

 now displayed both black and yellow molecules, the spines having 

 totally disappeared. 



As it now seemed only fair to Herr Zeiss' splendid " oil immersion " 

 -^q, the aperture of the achromatic condenser was reduced to 44°, its 

 axis being coincident with the optical. In place of using cedar oil 

 the glass was immersed in a mixture of Rangoon and olive oil. Long 

 straight chains of red beads appear shaded black at their margins, 

 and yellow ones dotted black and in a lower focal plane (fig. 10). 



Shaded ribs filled with spherules finely seen with l-16th imm. 

 Gundlach. The aperture of this glass was further reduced by using 

 only a five-inch tube, and separating the front lenses as much as 

 possible. This also corrected the spherical aberration (introduced by 

 the tube being shortened) as well as reduced the angular aperture. 



In these very difficult resolutions, skill, even in full practice, fre- 

 quently fails. 



Fortune may, however, surpass it, and I may record here a remark- 

 able instance of it : — 



Herr Beneche, of Berlin, has made for the writer a very excellent 

 glass, which, however, performs best with direct central light (1-1 6th 

 immersion objective). It will be now seen how important a factor in 

 delicate observations is the thickness of the " cover" glass.* 



Example 11. — Data. Beneche. Sleeting sky — same object. The 

 nose touches the cover ; slight action of the screw collar compresses 

 the spherules. The Podura oil, natural to these scales, is seen slowly 

 to permeate within the now flattened sac forming the scale. Hows of 

 molecules vanish one after another. Upon releasing gently the slight 

 pressure, they reappear at will one at a time. A molecule remaining 

 still in contact with the cover appears dark. They lie in rows, by 

 twos and threes, and here and there form V's by anastomosis. Placing 

 the correction collar at the most favourable point for finest definition, 

 intensely dark, almost jet-black, spherules appear like shot to occupy 

 each rib. But the scale appeared utterly spoiled by pressure. The 

 next day, however, it had entirely recovered its former appearance by 

 the force of capillary attraction. Again, the native oil was squeezed 

 gently along its many channels at a venture, by means of the fine 

 focal adjustment. 



* Beneche particularly insists, very properly, upon this : as, after all, the collar 

 adjustment is but a coarse compensation for the peculiar aberrations introduced by 

 a thin plate of glass of varying quality and thicknesSi 



