ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 775 



M. Chaudesaigues working with the same granules, found that the 

 following equation, also based on molecular hypotheses, was satisfied: 



where £ 2 is the square of the mean projection on an axis 0^ of the dis- 

 placement in a time t of a granule of radius a as a fluid of viscosity £. 

 In this case N works out to 70 x 10"-" 2 . 



The authors have repeated these experiments with granules of 

 mastic obtained by aqueous precipitation from the alcoholic solution of 

 this resin. The apparent density of these granules is only one-third of 

 that of gamboge, so that they formed a very suitable material for 

 further verification of the formulas. The layers measured were separated 

 by intervals of 6 yu. The granules were obtained after fractional cen- 

 fcrifugation, and their radii were found to be 0*52 /x. Evaluation of the 

 formulas gave results confirming those of the previous experiments, and 

 the average value for N was 70* 75 x 10 22 . Hence it follows that the 

 charge e of the electron is 4 - 1 x 10 - 10 electrostatic units. 



J. Perrin * has also attempted to verify by experimental observation 

 measurements the rotational equation given by Einstein,f 



a Rt 1 



" T N 4 7r£a 3 ' 



where £ denotes the viscosity of the fluid, a the mean square of rotation 

 of the granule in time t about an arbitrary axis ; R, T, N have the same 

 meanings as above. The author had some difficulty in finding granules 

 suitable for observation, but ultimately succeeded with a preparation of 

 urea. The theoretical value of N is 65 x 10 22 , and the experimental 

 value (after many observations) found was 70 • 5 x 10 22 . The conclusion 

 to be drawn is that the kinetic molecular hypothesis finds experimental 

 corroboration in the study of Brownian movement. 



c6> Miscellaneous. 



History of Optical Glass.} — M. von Rohr gives an interesting 

 sketch of various personalities connected with the great Jena industry 

 from about 1800 to the present time. The first names mentioned are 

 those of P. L. Guinand and his son A. Guinand, who, with J. Fraunhofer, 

 directed the manufacture till December 1813. A final separation 

 seems to have then taken place, when J. Fraunhofer went to Benedict- 

 tJeuren and the Guinands to France. G. Bontemps, who had gained his 

 experience with the French house, entered the service of Chance 

 Brothers, Birmingham, in 1848, and placed his knowledge of optical 

 glass at the service of his employers. A kind of genealogical table is 

 given showing clearly the connection of the various names with one 

 another. 



Practical Microscopy. §— The first edition of Shillington Scales' 

 Microscopy was so much appreciated that only four years have elapsed 



* Comptes Rendus., cxlix. (1909) pp. 549-51. 



t Ann. der Physik, xix. (1906) p. 371. 



% Zeitschr. f. Instrumentenk., xxix. (1909) pp.' 50-7. 



§ London : Baillere, Tindall, and Cox, 1909, xvi. and 324 pp. (122 figs.). 



3 F 2 



