168 Transactions of the Society. 



Upon the lines of the Ellis Aquatic MicroscojDe, nearly all later 

 models of dissecting Microscopes were fashioned. Owing to the 

 careful way in which the lenses are worked and mounted, and the 

 fine-adjustment supplied, it is possible to get a very good image 

 with this Microscope. (See Journ. E.M.S., 1898, p. 675 ; and 

 1909, p. 654.) 



12. Benjamin Martin (c. 1760). 



The maker of this instrument devised numerous improvements 

 in the mechanism and optical arrangement of the Microscope, and 

 the present model is an important link in its development. For 

 the first time there is a slow and fine movement for focusing, by 

 rack-and- pinion and by screw, both applied to the stage, with the 

 constant action of a spring to check the motion. A small compass- 

 joint at the top of the pillar allows the carrying-ring to be turned 

 out of the way when the instrument is used as a simple Micro- 

 scope. (See Journ. E.M.S., 1900, p. 269 ; 1909, p. 655.) 



This model is very interesting as, optically speaking, it differs 

 from any of the previously described Microscopes. The eye lens 

 is double (which gives a larger and flatter field), being composed 

 of two double convex lenses ; there is the usual field lens, and, in 

 addition, at the top of the nose-piece, there is a small biconvex lens 

 of about 5 in. focus, which can be used with any of the object- 

 glasses, and really forms a back combination for all of them. 

 Mr. Nelson has pointed out in an article describing a very similar 

 Microscope in his possession that the action of this lens by reducing 

 the power and increasing the aperture increases the optical index, 

 and consequently improves the Microscope. This important 

 improvement was later adopted by all the leading opticians, and 

 is found in most of the better-class Microscopes till the advent of 

 the achromatic form in 1826. 



13. Benjamin Martins " Large Universal Microscope " 



(c. 1771). 



This exceedingly elaborate instrument, of exquisite workman- 

 ship, with every conceivable movement, is said to have been made 

 for King George III. The meaning of the word "universal" is 

 that it can be used for viewing opaque or transparent objects with 

 either a single or double lens combination, that is to say, either 

 as a sim])le or as a compound instrument, and that it possessed, 

 in addition, the joints and accessories necessary to enable the 

 user to direct his gaze in a horizontal, vertical, or inclined direction 

 at his choice, and to carry the body of the instrument over the 

 different parts of the stage by what was then called the " aquatic " 



