138 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



scopical construction were here embodied. (1) A fine -adjustment 

 screw F is connected with the sliding socket E, supporting the arm 

 D, in which the body-tube is screwed ; the focussing could thus be 

 controlled in a far more effective manner than by any system pre- 

 viously applied to a large microscope. The previous systems involved 

 the direct movement of the body-tube either by rotating in a screw- 

 socket (as in Hooke's) or by sliding in a cylindrical socket (as in 

 Divini's and Cherubin's) ; in a few instances the object was moved 



FIG. 106. M. Joblot's microscope (1718). 



in relation to the object-lens, but all these plans were more or less 

 defective, especially with microscopes of large dimensions. Marsha 1 1 's 

 system was a distinct mechanical improvement, for the object could 

 now be viewed during the actual process of focussing, as the image 

 would remain steadily in the field. (2) A fork, N N, is here applied 

 with a thumb-screw clamp, O, on the pillar itself. (3) Hooke's ball- 

 and-socket joint, which was applied to the arm I, is here shifted to 

 the lower end of the pillar, where it would give the movements of 

 inclination to the whole microscope instead of to the body tube only, 



