I 



850 SITMMAKY OF CUERENT RESEARCHES RELATING TO 



at the edge of the cover has hardened somewhat, run a circle of a 

 solution of shellac in alcohol, so as to touch both the edge of the 

 cover and the slide. This will hold all fast, even though the balsam 

 be still liquid within. Finish this if you choose at once with tube 

 paints, and your slide is done. Examine the slide by oblique (black- 

 ground) light, or far better, if you have it, by polarized light. Use 

 the green, not the purplish coloured 'Wandering Jew.' You will 

 find the needles beautifully distributed, clean and looking like 

 polished steel." 



Preparing Crystals of Metals.* — Professor A. H. Chester gives the 

 following as his method of procuring crystals of the various metals : — 



" The beautiful and rare crystals of gold of hexagonal form are 

 prepared by first dissolving the gold in ten or twelve parts of 

 mercury, and boiling for several hours. The mercury is then 

 dissolved out by means of nitric acid, and the crystals of gold left 

 of hexagonal shape, their beauty depending upon the perfection of 

 the process. The silver, tin, copper, and other metallic crystals 

 are best obtained by precipitation of the crystals by means of the 

 battery. To obtain tin crystals the simplest method is to immerse 

 the end of a bar of tin in a strong solution of the chloride. A thin 

 layer of water is poured on the top. In a few days the crystals of 

 tin will be found attached to the bar in the layer of water." 



Blue Glass for Test Objects.f — E. Mauler mounts diatoms 

 intended as difficult tests on or under blue glass. The object is 

 twofold : — 1st, to render the image clearer by monochromatizing the 

 light entering the objective. In this case it is the cover-glass only 

 which is blue, and it " has the effect of improving the often con- 

 fused resolutions given by objectives whose chromatic aberration is 

 badly corrected." 2nd, by using blue glass for the slide, or for 

 the bottom of the cells, the light reaches the object monochromatised, 

 a plan which replaces the more inconvenient one with sulphate of 

 copper. Stronger illumination is of course necessary than with 

 ordinary glass. 



Fig. 200. 



Armstrong's Universal Turntable.— This instrument (Fig. 200) 

 is made by Messrs. T. Armstrong and Brother, of Manchester, and 



* Amer. Journ. Micr., vi. (1881) p. 125. 



t Joum. de Microgr., v. (1881) pp. 111-12, and Bull. Soc. Belg. Micr., vu. 

 (1881) pp. cxxxiii.-iv. and cxliii. 



