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



355 



pose of these diaphragms was for diminishing the spherical 

 aberration by cutting down the apertures of the observing lens 

 and not for regulating the illumination. The next model, that 

 of John Marshall, 1704, takes us on several steps in the evolution 

 of the microscope (Fig. 9). Here we first meet with the box-foot, 



Fig. 9. 



a distinctive feature which lasted for nearly 130 years. The 

 coarse adjustment is effected by a collar and jamb-screw sliding 

 on a square bar, the fine adjustment by a direct acting screw, /. 

 It is hardly correct to speak of the sliding arrangement as a 

 coarse adjustment because the post, a, was marked with numbers 

 corresponding with similar numbers marked on the objectives ; 

 the body remained clamped at the given mark until the objec- 

 tive was changed, all the necessary focussing being performed by 

 means of the direct acting screw. The great advance made in 

 this model consists in the pivoting of the lower end of the bar, 

 a, on a ball and socket joint, b. As the stage, d, is also fixed to this 



