ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 109 



metal reflector-diaphragm between them. The sizes and focal lengths 

 of the lenses are approximately as follows : Inequi-convex, diameter 

 1-3; focus 2*1. Plano-convex, diameter "65; focus l - 75. Focus of 

 combination, diameter 1*95. Of course, all these can be varied in 



Fig. 9. 



relative proportion if required. The three elements are mounted 

 separately, so that, although it is calculated to act as a " system," either 

 lens may be used by itself with or without the speculum. 



(2) Eye-pieces and Objectives. 



Nelson's Formula Oculars.* — A. A. C. E. Merlin calls attention to 

 the very fine visual results obtained by the employment of E. M. Nelson's 

 new formula Huyghenian eye-pieces when fitted to the telescope. One 

 of these yields a measured power of 160 diameters on a 3-3-in. clear 

 aperture refractor, the object-glass of which was made by Wray. The 

 formula of this description of eye-piece was computed by Mr. Nelson 

 some years ago, and is published in the last edition of Carpenter, but 

 its high qualities when used on an astronomical telescope do not appear 

 to be generally known. 



The author is satisfied of the superiority of these oculars for critical 

 microscopical work, over the compensated or ordinary Huyghenian eye- 

 pieces, when working with apochromatic, semi-apochromatic or achro- 

 matic objectives. 



Lens Calculation.! — " H." in a letter to the English Mechanic, 

 compiles the following bibliography of works useful for above purpose. 



1. The Perthensis Encyclopaedia, vols. xvi. and xxii. 



2. Encyclopaedia Britannica, third edition. 



3. The Telescope, by Herschel. 



4. Optical Instruments, by Herschel, in vol. ii. of the Library 



of Useful Knowledge. 



5. Rees' Cyclopaedia— very full and complete— vol. xxxv. being 



the most useful one. 



6. Coddington's Optics. 



7. Potter's Optics, part ii. 



8. Hansen's Dioptrische Untersuchungen. 



9. W. Scheibner's Dioptrische Untersuchungen. 



10. Steinheil's Handbuch der Angewandten Optik, containing 

 numerous worked out examples and figures. 



* English Mechanic, lxxviii. (1903) p. 425. 

 t Tom. cit., (Nov. 13, 1903) p. 316. 



