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



His first instrument was called the Pocket Reflecting Micro- 

 scope with a Micrometer, an account of which he published in 

 1739.* 



This instrument was dioptric ; the word " reflecting " applies 

 solely to the illuminating mirror, as was before noted in the case 

 of the Culpeper. Fig. 20 shows that it was of an entirely novel 

 design ; the focussing was performed by a push tube at B ; for illu- 

 mination the outer tube was cut away in front, and a mirror was 

 placed at the bottom ; there was also an aperture, P Q, cut through 

 the opposite side of the outer tube for the purpose of allowing sliders 

 to be passed through. One of the novelties this microscope 

 possessed was a rough micrometer fitted in the focus of the eye- 

 lens, with an attached dial for the purpose of registering the 

 tenths of a revolution of the screw. The price of this microscope, 

 including the micrometer, was .£1 Is., without the micrometer 

 IO5. Qd. Subsequently the outer tube was prolonged, making 

 what is now known as a drum foot, in which a mirror was placed ; 

 afterwards it was mounted on three legs after the manner of the 

 Culpeper. t This form of microscope has been largely employed, 

 principally on the Continent, and even now we still find 

 microscopes of this design in toy shops. The novelties in this 

 design are therefore the drum foot and the attachment of a 

 micrometer to the microscope. B. Martin has been held to have 

 been the inventor of the principle of microscopic micrometry, 

 but from what we have seen above he was clearly pre-dated in 

 this respect ; he was, however, the first to attach a micrometer 

 permanently to the microscope. 



Another instrument was designed by B. Martin in 1740, called 

 the Universal Microscope ; its form was too crude to call for 

 more than a passing notice. The foot was an octagonal box ; the 

 body, which had push-tube focussing, was attached to the top of 

 a pillar by a ball-and-socket joint ; the stage had transverse motion 

 in arc. This, in short, is a throw back to the telescope form of 

 mount ; but from about this time we shall see less and less of it, 

 because the invention of the mirror rendered the telescope mount 



* For this date see Adams on the Microscope, second edition (1798), 

 p. 21, footnote. 



f Both these improved forms are figured in Martin's Tracts, a book 

 in the Royal Microscopical Society's library. It is unfortunate that the 

 particular tracts which contain these figures are undated. 



