Vision and Definition under the Microscope. 173 



" Described and figured in Chevalier's " Des Microscopes et de 

 leur Usage," p. 88, pi. 4. (See Journ. E.M.S., 1909, p. 649.) 



The definition in this instrument is hardly inferior to that of 

 modern instruments with a 2-inch objective. The improvement 

 upon earlier models is chietiy noticeable with the higher powers. 



Chevalier added a set of rotating diaphragms, similar to those 

 now in use, to his simple Microscope, wliich was modelled on 

 Ellis's " Aquatic Microscope." 



22. Httgh Powell (18?j9). 



This Microscope is of verv great interest, because it embodies 

 new features which have now been very generally adopted in the 

 design of the Microscope. The body, stage and mirror are carried 

 by the limb, which itself is attached by a compass- joint to an 

 upright telescopic pillar raised on a solid tripod. The coarse- 

 adjustment by rack-and-pinion for the first time moves the body 

 of the Microscope, but the fine-adjustment is applied to the stage 

 by a wedge acted on by a micrometer screw. In this model also 

 Hugh Powell systematically applied the method of " springing " 

 in the movements to prevent loose action ; its application to the 

 pivots of the mirror can be well seen. 



Described in Journ. R.M.S., 1901, p. 728 ; and 1909, p. 659. 



It is of interest to note that the whole of P. H. Gosse's work 

 on the Eotifera was done with a Microscope of this type, which he 

 used throughout his life, " in spite of modern improvements." 



From this time the definition of low-power objectives has 

 altered very little. Until the introduction of the latest corrected 

 and apochromatic objectives in recent years there was but little 

 change, even in medium and high powers. 



23. JIugh Poivcll. Large Ificroscojjc (1841.) 



This almost too elaborate and substantial stand was considered 

 the best of its day, and embodies all the most refined movements 

 and apparatus the maker was able to devise. The body is moved 

 by rack-and-pinion, and is attached to a hollow triangular bar. 

 The fine-adjustment actuates the stage. Originally this was a 

 monocular Microscope, l)ut tlie binocular body with Wenham's 

 prism W9S fitted to it after the invention of the latter in 1863. 



Described and figured in Journ. E.M.S., 1900, p. 285 ; and 

 1909, p. 659. 



