98 



On the Evolution of the Microscope — Part 2. 



(Continued from page 356, Vol. 6, Ser. 2.) 



By Edward M. Nelson, P.R.M.S. 



{Talien as read, June 17 tJi, 1898.) 



Since the publication of Part 1 a microscope which contains 

 a very important feature has been found; as its date is 1702 it 

 should be inserted in Part 1 between Hartsoeker's and Wilson's 

 screw-barrel microscopes. Fig. 10,* which is a back view of it, 

 shows an oval w^ooden plate ; on the other side of this is a similar 

 plate, which holds the lens in such a position that it is opposite 

 the aperture A. Between the two plates there is a rotary 

 multiple object holder (MN, Fig. 11), the object being inserted in 

 apertures in the circumference of the disc. Focussing is accom- 

 plished by means of the milled head B, which is attached to a 

 screw which regulates the distance between the two plates, one 

 of which carries the lens, the other the rotary object holder. The 

 point of interest in this instrument is the rotating wheel of 

 graduated diaphragms A, C, D, E, placed, as you will observe, on 

 the side of the object remote from the lens. This is the first 

 instance where we meet with this useful appliance, which survives 

 to the present time. 



This pre-dates the loose diaphragms mentioned in connection 

 with the Wilson screw barrel, and is far superior, both from an 

 optical and mechanical standpoint, to Musschenbroek's sector of 

 diaphragms, placed in front of the lens. 



This microscope is similar in design, with the exception of the 

 rotating wheel of diaphragms, to an Italian microscope (date 

 1686), which had, in place of the rotating w'heel, a single 

 diaphragm in a disc Q pivoted at E, capable of being turned to 



* Copied from Zalm's Octdus Artificialis, 2nd Ed. (1702), page 796. 



