20 



Notes. 



on the top of the pillar ; this gives the lens a transverse motion in arc 

 across the object. The idea, in those days, was to move the lens over 

 the object, so that when infusoria were being examined they might 

 not be disturbed by the movement of the stage. 



llliliil'H'U 7 



d e 



ft :tl illlHIMIIII 



Fig. 17. 



This Microscope is a stage-focusser ; a rough adjustment is ob- 

 tained by sliding the mechanism on the rod by the hand, and then 

 clamping it by a pinching screw, after the manner employed in many 

 old Microscopes, J. Cuff's for example. The mechanical adjustment 

 is by means of an excentric strap on the head of the pinion, in fact, 

 similar to the coarse adjustment on the Plossl Microscope lately pre- 

 sented to the Society by Sir Ford North, the difference being that, 

 while the Plossl has a crank-pin and a connecting-rod, Holtzapffel's 

 has an excentric. 



The lenses were supplied by Andrew Pritchard. 



The date of this Microscope is 1830, and in it we find four original 

 devices: — (1) The clamp-foot; predating that of Varley's in 1831. 

 (2) The polished brass monochromatic mirror. (3) The focussing 

 movement by an excentric, which differs from and predates the some- 

 what analogous device of Plo?sl. (4) The extension movement of the 

 lens-holder by a steel tape and pinion. 



A Bibliography of Works {dated not later than 1700) dealing 

 icith the Microscope and other Optical Subjects. 



By Edward M. Nelson. 



Acta Eruditorum. (Leipsic, 1G8G.) 



Alhazen (1100): Thesaurus Optica?. (Translated from Arabic 

 Basil. 1572.) 



Aquilonius Franciscus : Optica. 



Bacon, Roger: Opus Majus Pt. V. De perspectiva. (1268.) 



Barrow, Dr. Isaac: Lectiones Optica?. (1669.) 



Bartholinus, Erasmus : Experimenta Crystal li Islandici. (Ha^as 

 Comit. 1669.) 



Bayer, John : Uranometria. (1603.) 



Bettimis, Marius : De Apiariis novissimi.*. 



