ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 363 



restricted to this application. The term " power " should be reserved 

 for the magnification produced at unit distance from the posterior focus 

 and at each successive unit. This unit distance should be in each case 

 the metre, or better, the decimetre, which corresponds more closely to 

 ordinary microscopical working distances. 



In a subsequent paper * the author discusses the methods of 

 evaluating the magnifying power. 



1. By direct measurement with a micrometer objective and a micro- 

 meter ocular, the latter being placed exactly at unit distances. The full 

 details he has explained elsewhere.! The sole inconvenience is that a 

 specially made tube is necessary, a decimetre tube not being of 

 ordinary make. 



2. By means f of graphical curves a relation may be expressed 

 between the power and the distance between the posterior face and the 

 posterior focus. This method readily gives the magnifying, power. 



3. By calculation from the formulas P= (1 — <fi' p ) y, or P= (1 + </>'„) y, 

 when P= the magnifying power; y, the power; <£',„ <j>' p , the distance 

 from the posterior face of lens to its posterior focus, according as the 

 posterior focus is on the anterior or posterior side. 



Evaluation of the Power of Microscope Objectives. §— L. Malassez 

 gives three methods by which observers may determine powers for them- 

 selves in his proposed notation. || 



1 . Let G be the magnification produced at a distance D, g that at a 

 less distance d, and y the power (i.e. the magnification produced at unit 

 distance), then 



_ G-// 

 y ~ D -cf 



In selecting the distances the tube should be completely extended for D 

 and completely depressed for d. 



2. Let g be the magnification produced by the objective at a distance 

 d' from its posterior face and <£' its foco-facial posterior distance (i.e. the 

 distance between its focus and posterior face), then 



the first formula being applicable when the posterior focus of the 

 objective is behind the posterior face ; the second, when it is in front 

 of it, which happens when the objectives are somewhat strong. 



If the magnifying power P be defined as on a previous occasion,T 

 these formulas become 



P P 



7-i-4>; ?- i +.<£'/ 



* Comptes Rendus, cxli. (1905) pp. 1004-6. 



t Archives d'Anatomie Micr., 1904, pp. 274, 285. See also this Journal, 1905, 

 p. 500. % Tom. cit., p. 298. See also this Journal, 1905, p. 500. 



§ Comptes Rendus, cxlii. (1906) pp. 773-5. 



|| Soc. de Biologie, viii. (July 15 and Dec. 10, 1904). See also Archives 

 d'Anatomie Micr., 1904, p. 270; and this Journal, 1905, p. 500. 



^[ Comptes Rendus, Nov. 27 and Dec 11, 1904. See also this Journal, 1905.. 

 p. 500. 



