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



Baker, in his description of this microscope in 1743, adds a 

 conical diaphragm made of black ivory (Fig. 17). A diaphragm 

 of this form made of brass was subsequently used by Cuff, 

 Benjamin Martin, Adams, and in 1798 by Jones. Chevalier 

 placed a graduated wheel of diaphragms at the lower end of 

 the cone (1823), so this peculiar form of diaphragm had an 

 innings of about eighty years. 



[With regard to the graduated wheel of diaphragms, it is 

 interesting to note that this useful appliance has not hitherto 

 been found in any microscope between the dates 1702 and 1823 ; 

 but I have lately acquired an old French microscope which 

 has the graduated wheel below the cone. The date of this 

 instrument is unknown, nor can it with certainty be deter- 

 mined, for the body has an archaic appearance, being something 



Fig. 17. 



like Joblot's compound microscope ; but as the stage has a 

 rack-and-pinion focussing movement, its date cannot be earlier 

 than about 1765. 



If, on the other hand, its date is prior to 1823, then the 

 microscope is especially interesting as being the first known 

 example of the revival of the graduated wheel of diaphragms.] 



The object-glasses, which were five in number, were called 

 '' buttons"; this term has now died out, but it was still employed 

 twenty years ago with reference to the French achromatic doublets 

 largely used by medical students. The Culpeper and Scarlet 

 model continued to be a popular form of microscope for seventy 

 years. 



In a fine old example, made by Nathaniel Adams, the legs are 

 of brass, the outer tube is shagreen, and the nose-piece wood, 

 the object-glasses also being mounted in wood. 



G. Adams also made these microscopes with the following 

 alterations : the legs below the stage were scrolled, and those 

 above it were of the form of vertical pillars ; the box foot was 



