ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1087 



The drum g through which the micrometer-screw passes is marked 

 by six lines of holes. A spring-catch supported by a vertical rod 

 fixed to / snaps as the drum revolves. 



Fixed to the extremities of a* by one end, and by the other 

 to a rod projecting from D, are two spiral springs for the pur- 

 pose of keeping up the tension of the carrier on the micrometer- 

 screw. 



If desired, an arrangement for fitting the instrument for cutting 

 under spirit can be applied, and also for raising the preparation 

 automatically. 



Efficiency of the Micrometer Screw.* — Prof. M. Gottschau in 

 reply to Herr Ost's paper,f repeats his previous conviction that 

 micrometers constructed with an inclined plane are not inferior, nay, 

 are superior, to those in which the motion is vertical or lateral ; the 

 chief points in favour of the latter are that the knife can be used in 

 its whole length, and that this construction is more convenient than 

 one which necessitates the constant whetting of the knife. With these 

 and other details, Prof. Gottschau does not agree. Dr. A. Brass, who 

 recently made some remarks j on the microtome knife and how to 

 manage it, also shares in the author's strictures. The result of the 

 matter simply is, that the one authority strops, and the other hones. 

 In this connection we may remark that nearly all microtomists seem 

 to differ on the treatment of knives, some advising soft stones, some 

 hard, and with or without the use of the strop. 



Use of Methylene-Iodide for Petrographical and Optical Pur- 

 poses^ — Herr E. Brauns directs attention to the value of this sub- 

 stance both as a liquid for the separation of minerals of different 

 specific gravities, and as a convenient medium for the determination 

 of refractive indices by the method of total reflection. For the former 

 purpose it is well adapted by reason of its high specific gravity (3 ■ 33), 

 which is greater than that of Thoulet's solution, and almost equal to 

 that of Klein's solution. The author finds the specific gravity to be 

 3*3485 at 5° C, and 3*3045 at 25° C, the variation being uniform. 

 For the successive separation of lighter minerals the liquid must be 

 diluted, not with water, but with benzole ; it may be readily concen- 

 trated again by distilling off the benzole, and is purified by shaking 

 with diluted potash water. 



For optical purposes it is particularly fitted by its high index and 

 by the fact that it is not decomposed or diluted by exposure ; in 

 these respects having a considerable advantage over Rohrbach's solu- 

 tion. The index of refraction for sodium light is 1 * 74873 at 5°, and 

 decreases uniformly to 1 * 73453 at 25°, while the decrease in the index 

 for each ray is equal for equal increments of temperature, but different 

 for different rays, the dispersion (which is considerable) becoming 

 less as the temperature increases. 



* Zeitsckr. f. Wiss. Mikr., iii. (188G) pp. 14-8. 



t See this Journal, ante, p. 538. 



1 Ibid., p. 706. 



§ Neues Jahrb. f. Mineral., Geol. u. Palaeont., ii. (188G) pp. 72-8. 



