Vision and Definition under the Microsco^Je. 169 



traversing motion. The triangular upright stem has a compass 

 joint at its base, and is fixed to an elaborate foot, over which it is 

 adapted to rotate. This foot is, in most descriptions of the instru- 

 ment, erroneously described as a tripod. The three feet do not, 

 in fact, support the instrument, but serve only to steady it, the 

 weight being carried by the knob in the middle of the foot. The 

 stage has micrometric movements in three directions ; it moves 

 the object over a wire scale in the eye-piece. This method of 

 micrometry was invented by Benjamin Martin, who also wrote a 

 book about it. The double mirror, as well as the stage, can be 

 raised and depressed by rack-and-pinion. The compound body 

 can be removed and replaced by a simple Microscope ; the stage 

 also can be removed and replaced. Provision is made for holding 

 and illuminating living objects and large opaque specimens. 



This Microscope is fully described in the Transactions of the 

 E.M.S. of 1862, p. 31. (See also Journ. E.M.S., 1909, p. 656.) 



The definition is improved by the back lens, and there is no 

 " fog " as in other contemporary instruments. The substage 

 condensers of Martin mark a great step in the progress of the 

 instrument. 



14. Brnjamin Martins " Neio Universal Microscope " 



(c. 1776). 



The stem inclinable by a joint at its base, where it is fixed to 

 ^. folding tripod foot. The body is attached to a movable arm, 

 which, in turn, is carried by the stem. This arm can be swung 

 about the axis of the stem, and moved to and fro in its socket, 

 these movements facilitating the exploration of a large specimen. 

 In consequence of this adaptation such Microscopes were, in the 

 latter half of the 18th century, called "aquatic." Focusing is 

 effected by rack-and-pinion, which moves the stem and hodj, 

 whilst the stage is fixed. This Microscope has a rotating multiple 

 lens-carrier nose-piece, invented by Pere Cherubini d'Orleans. 



Described and figured in Journ. E.M.S., 1899, p. 325. 



The improvements to this instrument are merely mechanical, 

 the optical construction not differing in any important particulars 

 from No. 12 before described. The adjustable pillar seen here was 

 first used in Watkins' Microscope (1754). The back lens is 

 absent and the definition is very poor. 



15. Shuttlewortli s Microscope (c. 1786). 



The stand of this instrument is a somewhat later imitation of 

 the Benjamin Martin type. The triangular stem has a compass- 

 joint at its base, by means of which the whole Microscope is 



