Francis Watkins' Microscope. By E. M. Nelson, 145 



most important, is the hinged limb which supports the Micro- 

 scope, the object, and illuminating apparatus ; the second, almost 

 as important, is the prism bar and V-grooves ; the third is the plane 

 and concave mirror. To this list may be added one of quite 

 secondary importance, viz. it is an early example of the improved 

 form of the wheel of powers. Permit me to express my thanks to 

 Mr. Underwood for so kindly sending his Microscope for examina- 

 tion.* 



Appendix. 



As regards the performance of old non-achromatic Microscopes, 

 it may be pointed out that empty magnification had its use in pre- 

 achromatic days, for it was by this means that aperture in a 

 dioptric Microscope was obtained. The method of making these 

 objectives was probably to open out the diaphragm until the image 

 just begun to show signs of becoming foggy ; it will be found under 

 these circumstances that a ^ in. will have a N.A. of about 0* 1, and 

 a -jJq one of about 0*2. 



Benjamin Martin's No. 6 measures - 0425 N.A. and 5*3 O.I. 



„ 1 „ 198 „ 2-5 „ 



It was mentioned above that when Martin's back lens was 

 inserted, the apertures would be slightly increased ; used thus, the 

 No. 1 will just resolve 15,000 lines, Grayson. When a compound 

 body is placed over a lens, the focus is lengthened and the aperture 

 reduced ; it was very probably on this account that many of the 

 old observers, without knowing the reason, preferred a " single " to a 

 " double " Microscope. 



* This Microscope was sold by J. C. Stevens, of King Street, Covent Garden, 

 Feb. 18, 1908, for 52 guineas. The price obtained was due less to the scientific 

 or intrinsic value of the instrument than to the hallmark, date 1754. — [Ed.] 



