1897] 



MICROSCOPICAL JOUKNAL. 



339 



stage condenser, to reduce its aperture when high powers 

 were used. This is the earliest notice of diaphragms for 

 regulating the illumination. 



In the year, 1702, we find a crude form of simple 

 microscope by Mussenbroek. The only point of interest 

 it possesses is to be found in a sector of graduated dia- 

 phragm holes. The purpose of these diaphragms was for 

 diminishing the spherical aberration by cutting down the 

 aperUires of the observing lens and not for regulating the 



illumifiation. 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, 

 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, f. It is hardly correct to speak of 

 the sliding arrangement as a coarse adjustment because 

 the post, a, was marked with numbers corresponding 



