ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



475 



calcareous rock from a mixture of finely-ground lime and clay, 

 making a kind of hydraulic cement, with which the diatoms may be 

 mingled. When this hardens, the sections may be cut, and isolated 

 by treatment with diluted hydrochloric acid. The large Pinnularia 

 is a good species to begin with. 



Zentmayer's New Centering Turn-table.* — The turn-table repre- 

 sented in fig. 75 is the invention of Mr. J. Zentmayer. The plan of 

 centering the slide is, it is claimed, quite original and perfect in its 



Fig. 75, 



results. The slide is placed so that its edges are in contact with the 

 two pins projecting from the face of the plate. A ring with an oval 

 inner edge is fitted to the periphery of the disk, in such a way that 

 by turning it the slide is grasped at the diagonally opposite corners 

 by the inner edge of the ring, and is thus centered longitudinally. 

 The two pins centre it the other way. The ring may be easily 

 removed, and spring clips substituted when desirable. 



Phosphorus Mounts. — It was recently stated | that diatoms 

 mounted in phosphorus solution cannot be kept for any time. This 

 is not so. Mr. J. W. Stephenson has slides mounted several years ago 

 (one in 1873), which are as good now as at first. All that is necessary 

 is to avoid long exposure to daylight which turns the diatoms an 

 opaque red. 



Styrax. — On testing this medium (as supplied by Allen and 

 Hanbury) with the refractometer, its refractive index is found to be 

 1*585 very nearly. It has so much colour that it is difficult to 

 determine the third decimal with accuracy. 



If we take the index of diatomaceous silex to be 1 • 43, and of 

 Canada balsam 1 • 52, it is seen that styrax gives a marked increase of 

 visibility over balsam, for while balsam is only 9, styrax is more 

 than 15. 



* Amer. Mon. Micr. Joum., v. (1884) p. 23 (1 

 t Engl. Mech., xxxix. (1884) p. 149. 



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