ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



277 



which rotates on an arm pivoting on a second arm which is clamped to 

 the stage. The first arm can be clamped to the second in any position 

 by the milled head screw shown in the woodcut. A piece of watch- 

 spring beneath the disc serves as a brake to steady the motion. When 



Fig. 42. 



Fig. 43. 



the arm is set excentrically, the apparatus enables the observer to search 

 over a fresh gathering of diatoms, &c. Other discs can be substituted 

 as desired. 



Apparatus for measuring very minute Crystals.* — Herr G. Latter- 

 mann has devised the small apparatus shown in fig. 43, for the measure- 

 ment on the goniometer of crystals 1/10 to 1/20 mm, in length, which on 

 account of their smallness could not be 

 adjusted on this instrument. 



The apparatus consists of a small hollow 

 cube of metal, and a jointed piece with axes 

 a and b. The crystal to be measured is 

 fastened on the point of a fine needle, with 

 somewhat stiff Canada balsam. By turning 

 the axis a, the edge between two faces of the 

 zone to be measured (or rather its horizontal 

 projection) is adjusted under the Microscope 

 upon the thread of the eye-piece running from 

 back to front, while at the same time the edge A B is directed on the 

 stage also from back to front. The effect of this movement is to bring 

 the zonal edge into a vertical plane parallel to a face of the cube. The 

 cube is then turned over on its left side, and the zonal edge (now in a 

 horizontal plane) is brought strictly parallel to A B, by turning about 

 the other axis h. If the apparatus be now placed on the goniometer 

 resting on a face at right angles to A B, and be centered, the crystal 

 will be in a suitable position for measurement. 



Electricity, Application of, to Microscopy. 



[Discussion by Dr. W. J. Lewis, Dr, L. D. M'Intosh, and Dr. W. M. Seaman.] 

 Froo. Amer. Soc. Micr., X. (1888) pp. 178-9 (1 fig.). 

 EoYSTON-PiGOTT, G. W. — The Anti-diffiraction Micrometer. 



[" In using spider lines a certain amount of diifraction confuses the measure- 

 ment. When a metallic obstacle is interposed, the irapingent rays of light 

 are dispersed in a fan-like form. It has occurred to me that a refracting 

 cylinder, on the contrary, would refract or inflect these rays inwards, pro- 

 ducing darkness. These principles are best iLustrated by optical diagrams. 

 The opaque jaws of the micrometer slides are edged with thin rods of glass, 



* Tschermak's Mineral, u. Petrogr. IMittheil., ix. (1887) p. 49 (1 fig.). 

 Zeitschr. f. Wiss. Mikr., iv. (1887) p. 542 (1 fig.;. 



Cf. 



