Vision and Definition under the Microscope. 165 



It is only applicable to objects mounted in the special Lieber- 

 kuhn manner. The definition is admirable, but the image is, of 

 course, very small. 



Class II. — The Old Compound or "Double" Microscope. 



The Compound Microscope, as at first constructed, consisted of 

 a single e3^e-lens, and a lens that did duty for the objective. 

 About 1663 the field-lens was first added and soon became 

 general. About this time also Microscopes were made with com- 

 pound eye-pieces and compound objectives, but they did not 

 survive, or at any rate become general, and it was not until 1759, 

 when B. Martin added the extra lens at the back of the objective, 

 that any improvement was made in the optical part of the instru- 

 ment in its dioptric form. 



5. John MarsltaU's "Double Microscope" 1690. 



The points to be noted in the instrument exhibited, which is of 

 earlier date than the one in the Society's collection, are (i) the pro- 

 vision of a screw for fine focusing, and (ii) the clamping of the stage 

 to the pillar. The ball-and-socket joint at the base of the square 

 pillar is typical of the earlier models ; in the later instruments 

 the ball-and-socket is replaced by a rigid pillar and a mirror is 

 added below the stage. The 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 instru- 

 ment, provided with an objective for forming an image of the 

 object and an ocular for viewing the image so formed. Con- 

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

 This instrument has been lent us for exhibition this evening by 

 Mr. George H. Gabb, a member of the Quekett Microscopical Club. 



In 1702, Zalm in his " Oculus artificialis " (p. 796) shows what 

 is probably the first Microscope with diaphragms. 



6. Culpeper's Compound Microscope (ante 1738). 



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 wi'h a ball-and-socket joint. 



(This instrument is described and figured in Mayall's Cantor 

 Lectures, 1885, p. 31. See Journ. E.M.S. 1909, p. 653.) 



The definition is very fair. 



April 19th, 1916 N 



