ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 653 



WILSON'S Screw-Barrel Microscope. Invented about 1702. 



Type — Simple Microscope. 



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. In the other case a rod, provided with forceps, 

 replaces the object-slide, and a lens-carrying arm enables the magnifier 

 to be suitably displaced for viewing an object held by the forceps. This 

 was a very popular model in the eighteenth century. 



See Phil. Trans., xxii. pp. 1241-7. 



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



Class 2. — Compound. (To illustrate the Old "Double Microscope.'''') 



JOHN MARSHALL'S " Double " Microscope. Invented about 1704. 



Type — Compound Microscope, uncorrected. 



Points to be noted : — (1) The provision of a screw for fine focusing 

 adjustment ; (2) the stage clamped to the pillar. Early models of this 

 make of Microscope had a ball-and-socket joint at the base of the pillar, 

 but no mirror as late as 1718. In the present specimen the ball-and- 

 socket is replaced by a rigid pillar, and the addition of a mirror and 

 modifications of the stage show that it is of somewhat later construction — 

 about 1744. 



This instrument was described in the original advertisement of it as 

 " John Marshall's New Invented Double Microscope for Viewing the 

 Circulation of the Blood." The word " double " here signifies that it 

 was a compound instrument provided with an objective for forming an 

 image of the object and an ocular for viewing the image so formed. 

 Concerning this instrument see Mayall's Cantor Lectures, 1885, p 37. 



CULPEPER'S Compound Microscope. Date, before 1738. 

 Type — Compound, uncorrected. 



It will be observed that this is a modification of Wilson's simple 

 Microscope. A body-tube of ivory, with draw-tube, is provided for the 

 purpose of transforming it into a compound instrument which is 

 ■mounted on a pillar with a ball-and-socket joint. The ball-and-socket 

 was a favourite mounting contrivance with the early makers. It was 

 fitted to Hooke's instrument, and was adopted in his earlier models by 

 John Marshall. Its use for this purpose has been now entirely 

 abandoned in favour of the compass hinge. 



This instrument is described and figured in Mayall's Cantor 

 Lectures, 1885, p. 34. 



