288 Transactions of the Society. 



that this Microscope had the weight of the body and the coarse 

 adjustment placed upon its fine adjustment, which was actuated by a 

 cone on a micrometer screw. (The advancing cone, for stage focus- 

 sing, was first described by Andrew Pritchard in his Micrographia* 



The limb of this Microscope could be rotated at the flanges, seen 

 just above the level of the stage ; this limb, with its coarse and fine 

 adjustment, proved unsatisfactory, and quickly passed away ; but the 

 most important part of the model, viz. the trunnion on the limb, 

 which quite displaced the compass joint, constitutes a real advance in 

 Microscope construction, and is now almost the only method in use, 

 both here and on the Continent. This plan, as we have seen above, 

 was due to George Jackson ; the double pillar was of course only a 

 matter of detail. The Microscope had a Turrell's stage with non- 

 concentric rotation. 



The next Microscope made by Messrs. Powell and Lealand (fig. 79) 

 is a very important one ; it is figured and described in the November 

 number of the London Physiological Journal for 1843 (only five 

 numbers of this very rare work were published). We notice that both 

 the Lister limb with the Jackson groove and the flat tripod with the 

 two pillars have been discarded, and in their place we have a bar 

 movement and a true tripod to carry the Microscope. Inside the bar 

 or transverse arm is a lever of the first order, which moves only the 

 nose-piece carrying the objective, the other end of the lever being 

 actuated by an advancing cone on the end of a micrometer screw ; the 

 stage is similar to that of the preceding Microscope, viz. a Turrell's 

 with non-concentric rotation. The instrument is still supported on 

 trunnions, so the advantage of Jackson's plan is retained, although 

 his form of foot is altered. There can be no doubt that Microscopes 

 built on the bar movement model had far superior fine adjustments to 

 those made with a Lister limb, which at that time, and for forty years 

 afterwards, were fitted only with short lever fine adjustments attached 

 to the end of their body-tubes. 



As the publication of this Microscope pre-dated that of Andrew 

 Eoss by one month, it follows that the credit of the invention must 

 be given to Powell. In the article " Microscope " in the Penny 

 Cyclopaedia, 1839, Andrew Boss utterly condemns the bar movement ; 

 nevertheless in 1843 he adopted it, and it was exclusively used by 

 that firm for about thirty years. 



In 1847 Messrs. Powell and Lealand greatly improved this fine 

 adjustment by suppressing the advancing cone, and by placing a direct 

 acting micrometer screw in a vertical position on the top of the bar, 

 immediately behind its pivot (fig. 80) ; and from that day to the 

 present time the fine adjustments made by this firm have remained 

 unchanged. This plan of fine adjustment is first figured and described in 

 connection with a portable Microscope in the first edition of Quekett, 

 p. 80, fig. 45 (1848). Quekett, in his second edition (1852), p. 77, 



* Page 217, fig. 23, 1837. 



