Vision and Definition under the Microscope. 171 



17. Amici's Reflecting Microscope (c. 1827). 



This Microscope is achromatic, and there is therefore a distinct 

 improvement in the definition and in the image. (See Journ. 

 R.M.S., 1609, p. 120.) 



18. Cuthbcrt's Bcficcting Microscope (c. 1827). 



The attempts made at the close of the 18th and beginning 

 of the 19th centuries to produce achromatic object-glasses for 

 the Microscope having failed, owing to technical difficulties, the 

 makers of these Microscopes attempted to produce achromatism 

 by means of mirrors, carrying into effect a suggestion originally 

 made by Xewton, which one or two other makers had followed 

 up. Cuthbert's instruments are said by Mayall to have been the 

 best of their type. The magnification of objects is here effected 

 by means of very small reflecting specula, and the result for low 

 and medium powers was fairly satisfactory. The body is fixed 

 by a compass-joint on the top of the telescopic stem supported on 

 a folding tripod. The focusing is effected by moving the stage, 

 and the latter has rectangular motion. 



Described and figured in Mayall's Cantor Lectures, 1885, p. 58. 

 (See Journ. R.M.S., 1909, p. 657 ; and 1913, pp. 98-100.) 



Cuthbert's reflecting Microscope was said by contemporary 

 users to have been a great improvement on Amici's type. The 

 instrument requires some adjustment ; it is suggested that very 

 likely the figure of the mirror has been spoiled by repolishing 

 at some time, but this cannot be ascertained without making 

 a thorough test. 



•^o* 



Class V. — Achromatic Microscopes. 



19. Ddlcbarres " Mic7-oscope Universcl" {Uncorrected) 



(c. 1777). 



This stand is a French model on the lines of the English 

 Microscopes of the period. The square limb is fixed on to a 

 scrolled folding tripod foot, and has a hinge about its middle by 

 means of which the upper part can be inclined. The body is 

 fixed to an arm which slides in a rotating socket at the top of the 

 limb. The concave mirror and condensing lens slide on the limb. 

 The arm carrying the stage has a pinion moving in a rack cut in 

 the limb for the purpose of focusing. 



Dellebarre endeavoured to obtain achromatism by the use of 

 oculars built up of crown and flint glass lenses, the excessive 



