of the Objective Glasses of Microscopes. 49 



hundred to fifteen hundred diameters, with an ocular mag- 

 nifying ten times ; Ross's was the feeblest, that of Spencer 

 the strongest. 



The angular opening was first measured with great accu- 

 racy, and found as follows : — 



Ross, 145 



Spencer, .... 135 

 Nachez, .... 120 



These measurements were all verified by the respective 

 owners of these lenses. 



The objects examined were the most difficult test-objects 

 among the silicious infusoria, as the Navicula angulata, one 

 of the species of Gramatophora, and a Navicula called the 

 Amici test. The first two were in balsam. 



The lenses were first attached to one of Nachez' s mount- 

 ing, and the best adjustment of oblique light used that this 

 instrument affords. The difference in the effect of the three 

 lenses was very slight, all failing to show the lines on the 

 Gramatophora, or on the Amici test. As, notwithstanding 

 the admirable arrangement of Nachez's instrument for work- 

 ing purpose, we do not get the extreme obliquity of light 

 which is required for examining their fine lines, I had them 

 all arranged in a mounting of Amici, which furnishes the 

 necessary obliquity of light. Thus arranged, the lines on 

 the Gramatophora were distinctly and beautifully seen by all, 

 with slight advantages in favour of Spencer and Ross, the 

 former of which magnified them most. 



The Amici test was next tried, which resulted in Ross 

 showing the lines with perfect satisfaction ; Spencer shewing 

 them, but not quite so well ; Nachez still less distinctly. 



I would remark that this difference between the lenses 

 appears to be owing entirely to difference in the angle of 

 opening ; for where a very oblique light is necessary to shew 

 lines, the lenses must be so constructed as to admit this light. 

 I would also state that Nachez's system lacks an adjustment 

 which the others have, by which the relative position of the 

 lenses can be changed, so as to compensate for the thickness 

 of the glass which covers the object, and which appears 



VOL. LI. NO. CI. — JULY 1851. D 



