ZOOLOGY AND BOTANY, .MICROSCOPY. ETC. 765 



(5) Microscopical Optics and Manipulation. 



Drude's Theory of Optics.* — This important work has been trans- 

 lated from the German into English by C. R. Mann and R. A. Millikan. 

 The preface to the English translation has been written by Prof. 

 Michelson, who states that there is no other book in English which 

 embodies the important advances in both theory and experiment made 

 during the last decade. It excels in presenting a complete development 

 of the electromagnetic theory of light in all its bearings, and a com- 

 prehensive discussion of the relations between the laws of radiation and 

 the principles of thermodynamics. 



The book consists of three parts, respectively devoted to : (i) Geo- 

 metrical Optics, (ii) Physical Optics, and (iii) Radiation. 



Part i. is on the usual lines, and follows closely Czapski's treatment 

 in Winkelmann's Handbuch der Physilc. 



Part ii. is subdivided into two parts : general properties of light, 

 and optical properties of bodies. This part includes, as an important 

 advance upon most previous textbooks, Sommerf eld's rigorous solution 

 of the simplest case of diffraction, Cornu's geometric representation of 

 Fresnel's integrals, and, on the experimental side, Michelson's echelon 

 spectroscope. It also extends the hypothesis as to the nature of light. 

 The mechanical theories are merely mentioned, but the electromagnetic 

 theory, which the author considers to present the simplest and most 

 consistent treatment of optical relations, he has discussed at length. 



Part iii. is concerned with the relation of optics to thermodynamics, 

 and (in the third chapter) to the kinetic theory of gases. 



Numerical Aperture and Rapidity .f — W. A. E. Conrady in a paper 

 on this subject remarks, that whereas Microscope lenses are classified by 

 their N.A., photographic lenses are by their /-values, i.e. //8, //16, &c. 

 Formulas are given : first, to express the N.A. of a Microscope-lens in 

 terms of photographic /-value ; secondly, for converting photographic 

 /-values into N.A. 



I. /'-value = ^ T ■ . 



J 2 N.A. 



II. N.A. = 



2 (m + 1) X /-value 



N.A. is numerical aperture, and m magnifying power. 



Specific Double Refraction of Plant Tissues.:]: — B. Remec, after 

 many experiments, concludes : (1) That lignin has no influence on the 

 specific double refraction of plant tissue ; (2) that even in cell-walls of 

 similar thickness and similar chemical composition, the degree of double 

 refraction may vary according to peculiarity of organisation ; (3) that 

 if pores exist in the cell-wall, the greatest optical axis of elasticity of 

 Fresnel's ellipsoid lies in the direction of the pores ; (4) the membranes 

 of superposed tissues generally produce elliptic polarisation, the main 

 .axis being sometimes parallel, sometimes perpendicular to the anatomical 



* Longmans, Green & Co., London, 1902, 546 pp. (110 figs.). 



t Knowledge, xxvi. No. 216 (1903) p. 236. 



% S.B. Akad. Wiss. VVien., pp. 364-87 (3 figs.). R 



Dec. 16th, 1903 3 d 



