E. M. NELSON ON EVOLUTION OF THE MICROSCOPE. 



99 



one side out of the axis, Fig. 11.* This is the first notice of a 

 diaphragm for the purpose of regulating the illumination. The 

 rotating disc MN is better seen in Fig. 11, and the focussing 

 arrangement in Fig. 10. 



Dr. Smith, who in his CompUat System of OjJticks (1738) 

 gives an excellent and precise account of the microscopes of his 



Fig. 11. 



Fig. 10, 



day, and the way to use them, makes no mention of diaphragms. 

 He says, '' In the viewing objects, one ought to be careful not to 

 hinder the light from falling upon them, by the* hat, peruke, or 

 any other thing." With regard to the condensing lens of the 

 Wilson screw barrel he says : " The only use of the convex lens at 



* From JVuove inventiont .di^tubi ottici (1686): see Mayall's Cantor 

 lectures, Dec. 7th, 1885, p. 26. 



