142 Transactions of the Society. 



The measured foci of Watkins' seven powers* are as follows 



The powers with the compound body attached would, therefore, 

 range from about 30 to 430 diameters. Nos. 5 and 6 obviously 

 have been transposed. There are three lieberkuhns, diameters — 

 1 • 3 in., focus ' 6 in. ; 1*1 in., focus • 4 in. ; 0*8 in., focus • 3 in. 



This is an improvement upon Lindsay's plan of a single conical 

 speculum, which had to do duty for all the powers. Dr. Lieber- 

 kiihn's compass Microscope, made by Cuff (1743) had a separate 

 spherical mirror adjusted to each of its four powers, thus pre-dating 

 Watkins'. The body of Watkins' Microscope is 6 in. long, 1^ in. 

 diameter at its widest part, and elegantly tapered. Adams' " New 

 Universal " (fig. 27) is probably the earliest Microscope to possess 

 a body with this kind of taper. This taper survived a long time, 

 for it is found in Coddington's Microscope of 1830, t and in 1843 a 

 remnant of it is left by Hugh Powell at the bottom of the tube ; J 

 Beck and Eoss never tapered the body, but the Lister-Tulley, made 

 by Smith in 1826, was tapered at the bottom ; so tapered bodies 

 lasted about 100 years. 



The eye-piece is Huyghenian, and a very good one ; the eye- 

 lens is a plano-convex of 1 in. focus, and the field-lens an equi- 

 convex of 2 in. focus, the distance between them being If in. 

 Calculation shows that to obtain the best results the eye-lens 

 ought to have a focal length of 0*865 in., and the distance between 

 the lenses ought to have been 1 • 785 in., so the old eye-piece is not 

 so far wrong after all. 



The fine-adjustment screw, which is placed at the bottom of 

 the limb, has 30 threads to the inch. This position for the fine- 

 adjustment screw is derived from Adams' " New Universal 

 Double " (fig. 27) ; the difference between them should be noted, 

 Adams' at the bottom of the pillar, Watkins' at the bottom of the 

 limb. There is an old Microscope in the Society's cabinet with the 



* Culpeper and Scarlet's Microscope had five powers ; Wilson's screw barrel 

 si*x powers, foci 0" 5, 3, 0-16,0-08, 0-05,0-02. Lieberkiihn's compass Micro- 

 scope, made by Cuff (1743) had four powers, foci 1-0, 0-6, 0*3, 0-08. A Benjamin 

 Martin (circa 1760) has six powers ; their measured foci are as follows : 1 • 25, - 96, 

 - 46, 0-37, 0'31, - 13. The highest power was always numbered 1. It is curious 

 to note that the screw-thread of the " pipe " in Benjamin Martin's Microscope is 

 almost identical with that of the Society's standard thread — it readily screws on 

 the nose-piece of any modern Microscope ! 



t Coddington's Optics, pt. ii., pi. 13, fig. 190. See this Journal, 1898, p. 474, 

 fig. 82. This is Gould's Pocket Microscope {1828), made by Cary, 181 Strand. It 

 is very similar to Coddington's, the foci and lens distances are the same, but the 

 lenses, for cheapness (it may be presumed), are all equi-convex. 



t See this Journal, 1900, p. 289, fig. 79. 



