164 Transact/tons of the Society. 



The object is quite recognizable, although the definition is extremely 

 poor. This lack of definition is probably due to the fact that the 

 model-maker concentrated his skill on the reproduction of the 

 metal work and neglected the reproduction of the lens. 



2. Wilsons Screw-Barrel Microscope (1702). 



Two specimens of this instrument are exhibited, the one 

 showing its adaptation to the examination of transparent objects, 

 the other, its use with opaque objects. In the one case the object- 

 slide is held by a spiral spring and focused by a screw-barrel, 

 which also carries an illuminating lens. At the further end, in 

 the other case, a rod, provided with forceps, replaces the object- 

 slide, and a lens-carrying arm enables the magnifier to be suitably 

 placed for viewing an object held by the forceps. This was a very 

 popular model during the 18th centurv. (See Phil. Trans, xxii. 

 pp. 1241-7.) 



Described and figured in Journ. K.M.S., 1905, p. 740. 



The progress of vision and definition is of course inseparably 

 connected with the development of substage mirrors, condensers, 

 and diaphragms. These are indicated in Hooke's Microscope in 

 1665, and Bonanni in 1692 shows the first compound focusing 

 substage condenser, whilst in 1693 John Marshall, here in England, 

 had supplied a substage condenser to his Great Double Microscope, 

 consisting, of a convex lens at the end of a pointed arm which 

 could be adjusted up or down if required. In 1694 Hartsoeker 

 figures a single convex lens focusing by a screw, and in the 

 Microscope we are now showing there is a fixed convex lens, not 

 capable of focusing or adjustment. 



3. Johlot's " Troisieme Nouveau Microscope " (c. 1716). 



This instrument, which is the only one of its kind known to 

 us, is of very decorative and elaborate workmanship, but it is 

 unfortunately not in working order. Joblot's Microscope had a 

 substage diaphragm lined with black velvet which he called the 

 "canon." (See Journ. E.M.S., 1914, p. 297.) 



4. Lieberkuhn's Microscope, invented 1738, made by Benjamin 



Martin about 1762. 



This is a simple biconvex lens, mounted in the central aperture 

 of a polished metal reflector, which formed a small hand-Micro- 

 scope for the special purpose of viewing opaque objects. This 

 construction, in an impro\-ed form and applied to achromatic 

 object-glasses, is in use at the present day. (See Adam's " Micro- 

 graphia Illustrata," 1747, p. 16, and Journ. E.M.S., 1909, p. 652.) 



