ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
437 
1841 Mr. James Smith graduated the collar, and in 1871 (published 
1873) Dr. R. L. Maddox caused the mechanism to actuate the back 
lenses instead of the front, which is the form of correction at present in 
use. 
Now, if we turn to the figure (fig. 103) we shall find a very early 
and primitive form of correcting lens. The tube carrying the front 
Fig. 102. 
lens slides on an inner tube ; it can bo clamped in any position by k the 
screws at the sides ; the line in the little hole in the front indicates its 
position, and is the prototype of the “ covered ” and “ uncovered ” lines 
of later times. This interesting and very rare object-glass can safely be 
dated 1838. (The larger cylinder at the base is the lid of its box upon 
which it is standing.) 
Effect of Cover-glass Thickness on the Performance of Wide 
Aperture Dry Objectives.* — Mr. F. J. Keeley, of Philadelphia, in 
discussing this subject, points out the well-known fact that even an 
adjustable objective will do better work at certain cover-glass thick- 
nesses and tube -lengths than at others. This frequently arises from the 
fact of the adjustment having been made for a cover of greater thickness 
than the one actually used by the observer. In such cases another cover 
of suitable thickness attached by a drop of immersion oil to the fixed 
cover will procure a resolution of the object. 
Nomenclature of Objectives. — Mr. F. Howard Collins, F.R.M.S., 
sends us a communication on this subject, containing a proposal, on 
which he invites discussion. After pointing out how little the true 
character of objectives is described by the present system of desig- 
nating them alphabetically — a system whose value differs with every 
maker — he recommends an extension of the notation now so success- 
fully used with eye-pieces. His proposal is to call them by the initial 
* 
2 G 
1899 
Micr. Bull. Philadelphia, 1899, pp. 12-13. 
