146 ANNUAL OF SCIENTIFIC DISCOVERY. 



again inverted in the same plane by the second ; and is, therefore, 

 correctly represented in the drawing. 



Sir David Brewster, after expressing his approval of these simple con- 

 trivances, said, that there were physiological reasons which rendered 

 these contrivances for enabling a person to use the microscope with 

 erect head important. When the eye was turned downward, in the 

 first place, the fluid which works the cornea, and which during ordi- 

 nary vision is spread in a uniform film over the cornea by the action 

 of the cornea, and is constantly draining downwards over the cornea in 

 the intervals, collects, when the eye is placed downwards, in a lentic- 

 ular shaped mass, on the very centre of the cornea, so as greatly to 

 impede vision ; and, moreover, those little fragmentary portions of the 

 crystalline lens which, when it is breaking up, particularly in old age, 

 become the elements of the musca* volitantes, those which, in the erect 

 position of the head, by sinking down to the lower part of the lens, re- 

 main without interfering with vision, these, when the eye is turned 

 down, collect in Avhat is then the lowest and central part of the lens in 

 the direct line of sight, and greatly impede the rays of light. London 

 Athen&um. 



COMPARATIVE VALUE OF DIFFERENT MICROSCOPES. 



DR. J. LAWRENCE SMITH communicates to Silliman's Journal the 

 result of a careful examination of three microscopes, the production of 

 three of the most eminent manufacturers of the present day, viz. : 

 Spencer, of the United States, Ross, of England, and Nachez, of France. 

 The examinations were made at Paris, by a number of distinguished 

 microscopists, with a view of testing the value of the respective instru- 

 ments. Their magnifying powers varied from thirteen hundred to fif- 

 teen hundred diameters, with an ocular magnifying ten times ; Ross' 

 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. The objects examined were the 

 most difficult test objects among the silicious infusoria, as the Navicula 

 angulala, one of the species Gramatophora, and a Navicula called the 

 Aniici test. The lenses were first adjusted 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. With a better obliquity of light, in a different mounting, the 

 lines on the Gramatophora were distinctly and beautifully seen by all, 

 with slight advantages in favor 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. The 

 difference between the lenses appeared 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 bo so constructed as to admit this light. For 

 the examination of globules, no appreciable difference between the dif- 

 ferent lenses could be noticed. M. Nachez deserves mucli praise for 

 the manner in which he has improved the microscope in France with 



