32 fi • The Microscope. 



accurately focussed by means of the paper label on the test 

 slide. No sub-stage helps of any kind were used. After adjust- 

 ment of objective and light as described, an ordinary bull's 

 eye, same height as flame and 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^ inches, measured from its 

 lower end to top of draw tube, and could be elongated to 13 

 inches. The Tolles' and Spencer's objectives have screw col- 

 lars ; these were adjusted for their best effect with tube length 

 of 10 inches, 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, hut as difficult as any I have 

 seen in Mtiller's test slides. No. 19 is probably proportionally 

 easier often showing by light and adjustments which 18 defies. 

 I think this last is unusually difficult, aild the same ma}- be said 

 of No. 12, Gr^amatojyhora aubtilissima. The others seem to be 

 fair, average shells. As immersion media, somewhat thickened 

 cedar oil, as furnished by Zeiss, and a fluid sent out by the 

 Gundlach Optical Co., were used successively with all the ob- 

 jectives, 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 Amphi- 

 pleura 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-ad- 

 justable objectives did best with the shortest tube and negative 

 ocular. With Zeiss' compensating ocular, the result was rather 

 more satisfactory with the ten inch 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 short tube. 

 With tlie apochromatic at its best, the Diatom appeared perfectly 

 flat with mid-rib and margins, showing distinct and clear, when 

 the lines were in focus, a thing none of the others did, though 



