158 Miscellaneous. 



very creditable to himself. Their magnifying powers varied from 

 thirteen 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 accuracy 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 

 siliceous infusoria, as the Navicula angulata, one of the species of 

 Grammatophora, and a Navicula called the Amici test. The first two 

 were in balsam. 



The lenses were first attached to one of Nachez' s mounting, 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 Grammatophora or on the Amici test. 

 As notwithstanding the admirable arrangement of Nachez' s instrument 

 for working purposes, we do not get the extreme obliquity of light 

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

 ranged on a mounting of Amici, which furnishes the necessary obli- 

 quity of light. Thus arranged, the lines on the Grammatophora 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 showing 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 show lines, the lenses must be so con- 

 structed as to admit this light. I would also state that Nachez's 

 system lacks an adjustment which the others have, by which the re- 

 lative position of the lenses can be changed, so as to compensate for 

 the thickness of the glass which covers the object, and which appears 

 favourable to the examination of those delicate tests. For the ex- 

 amination of globules we could not perceive any appreciable difference 

 between the lenses. 



I would here remark in justice to M. Nachez that he deserves much 

 praise for the manner in which he has improved the microscope in 

 France, without augmenting the cost of the instrument, and out of 

 England he is undoubtedly the best maker in Europe. To furnish an 

 idea of what he has done to diminish the cost of a good instrument, I 

 will compare the price of the objectives which have been the subject 

 of the experiments. 



Ross 306 francs. 



Spencer 230 „ 



Nachez 60 „ 



