E. M. NELSON ON THE EVOLUTION OF THE MICROSCOPE. 



353 



design, which was crude in the extreme. It is indeed difficult 



to understand how the discoveries he made could have been 



carried out with such rude apparatus. 



In 1687 we find a microscope by Grrindl 

 very similar to Fig. 5. The optical part, 

 however, consisted of three pairs of plano- 

 convex lenses. 



In 1691 several new features appear. 

 Fig. 6 shows a screw-barrel compound 

 microscope by Bonanni. The slider 

 placed between two plates pressed to- 

 gether by a spiral spring, was made to 

 approach or recede from the objective 

 by a screw. This simple arrangement, 

 known as the "screw barrel," played 

 an important part in the history of the 



microscope for upwards of 100 years. 



To Bonanni we are also indebted for a horizontal microscope in 



1691 (Fig. 7). This instrument is noteworthy, first for the 



Fig. 7. 



double support to the body. A glance at Hooke's (Fig. 3) will 

 convince anyone how rickety the body must have been when 

 only held by its focussing screw, so here we have a decided im- 

 provement. Secondly, we have a rack, i, and pinion, h, coarse 

 adjustment, in addition to the usual screw fine adjustment, m, 

 of that period. There is also an improvement in the stage, and the 

 Journ. Q.M.C., Series II., No. 40. 25 



