ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



253 



he must correct them by spectacles. The two images are then identical 

 and the pointer appears in each. The slight difference in brightness is 

 not found to be a detriment. The author has, after a year's experience, 

 found the instrument a very valuable help in his demonstrations. 



Boegehold, H. — Eine neue Konstruktion von Korrektionslinsen. 

 [Chiefly relates to telescopes ] 



Zeit.f. Instrumcntenkunde, xxx. (1910) pp. 302-7 (1 fig.). 



(3! Illuminating- and other Apparatus. 



New Method for Microscopical Metallography.* — E. Sommerfeldt 

 describes J. Konigsberger's apparatus and method. If natural light be 

 allowed to fall by means of a vertical illuminator on an isotropic surface 

 the reflected light does not exhibit polarization. 

 But if the reflexion occurs at an anisotropic surface 

 a separation of the light takes place into two com- 

 ponents vibrating perpendicularly to each other, and 

 these components are of unequal intensity, so that 

 a partial polarization follows. Two arrangements 

 serve f or realizing this polarizing effect on well-polished 

 surfaces. The simpler arrangement — not serviceable, 

 however, for quantitative measurements — consists of 

 a Klein quartz plate, in combination with a nicol 

 prism (polarizer), placed in front of the vertical illu- 

 minator ; the analyser (inner nicol) remaining in the 

 usual place. The Klein plate gives violet effects with 

 isotropic substances ; but with anisotropic substances 

 it furnishes a coloured field which changes during 

 rotation (red and blue; with strong anisotropy, bright 

 yellow or green). The second arrangement involves 

 the use of a Savart's double plate, which, observed 

 with a telescope adjusted for infinity, must reveal 

 between crossed nicols two deep black quite sharp 

 bands, surrounded by coloured bands. If unpolarized 

 light is reflected (i.e. if the preparation is isotropic) 

 no bauds appear. The more complete the polariza- 

 tion of the reflected light the clearer appear the bands, and therefore 

 the deeper the anisotropy of the preparation. By means of a contrast 

 plate, prepared out of two smoke-quartz "plates cut perpendicularly to 

 the axis, the bands become intensified. The preparation must be very 

 accurately set perpendicular to the incident light rays, and this is effected 

 by the adjusting apparatus J seen in fig. 35. 



Application of Mercury Light to Microscopical Works.f — A. Kohler 

 describes a Hageh Microscope lamp which the Zeiss firm have made for 

 him and which he has found very useful in his microscopical work. The 

 light source is furnished by one of Messrs. Schott's Hageh lamps, in 

 which the mercury column has a special length of 20 cm. A special 



* MetaUurgie, vi. (1909) pp. 605-7 (1 pi. and 1 fig.) . See also Zentralbl. f.Min., 

 Geol. u. Palaont., 1908, p. 565 ; 1909, p. 245; and Zeitschr. wiss. Mikrosk., xxvii. 

 (1910) p. 445(1 fig.). 



t Zeitschr. wiss. Mikrosk., xxvii. (1910) pp. 329-35 (1 fig.). 



April 19th, 1911 s 



Fig. 35. 



