TuE Microscope. 235 



end of tube, at its shortest length, is seven inches, at the longest 

 ten inches. Zeiss says the objectives for short tiibes are corrected 

 for 160 m m., or abont six and four-tenths inches. Trial now imme- 

 diately revealed the fact that a mistake had been made, and that the 

 objective was really intended for the short tube. I had no further 

 difficulty in neatly resolving balsam mounted Ampliipleura without 

 interposing anything between the mirror and the slide. The lines 

 showed throughout the length of the frustule and distinct enough 

 for anyone accustomed to microscopic work to easily see them. By 

 the aid of a micrometer I think I could have accurately counted 

 them, though I did not try. The effect of elongating the tube was 

 clearly perceptible with both central and oblique light. The correc- 

 tion is certainly for the short tube, and mine being still a little too 

 long the best results were not attained. 



I next tried a Zeiss stand, but unfortunately could' not use the 

 new eye pieces, these being mounted for the larger tubes. For this 

 kind of work the mirror is an awkward affair. In fact I could do 

 nothing satisfactory with this stand (Stativ I) in the matter in hand, 

 without the Abbe condenser, and with this I could not compare the 

 results obtained without sub-stage apparatus. But it speedily be- 

 came evident that the objective did its best work at a tube length of 

 160 mm., thus confirming the conclusion before reached, that a 

 mistake had been made in engraving the objective and in filling my 

 order. 



After gaining some familiarity with the objective and the ocu- 

 lars, an attempt was made to more carefully note just what could be 

 accomplished and how the best results were attained. A slide of 

 Amphipleura peJlucida from Lake Pistakee, Wis., mounted by B. 

 W. Thomas, of Chicago, in balsam, was chosen because the frustules 

 are the most difficult of resolution of any in my possession. That 

 on my Moeller's test plate is easy compared with them. I used a 

 common coal -oil lamp with No. 2 burner, with the chimney smoked 

 inside except a narrow space directed towards the mirror, the flame 

 edgewise, turned rather low. The lamp set at right side thirty to 

 thirty-four inches from mirror. The latter was carefully focused by 

 the use of a strip of thin paper held on the slide before the objec- 

 tive was brought down. This focusing was attended to every time 

 the obliquity of the mirror- bar was changed. Bullock's stand with 

 short tube as described was used. After thiis adjusting the light, if 

 further illumination was desired a bull's-eye was placed as near the 

 chimney as })racticable and moved to proper position. 



