ZOOLOGV AND BOTANY, MICROSCOPYj FTC. 62] 



a drop or two of water containing Infusoria. The cotton is thon spread 

 out into a thin layer about the size of the cover-glass to be used. The 

 cover is then placed on the cotton layer and secured in position by 

 means of rubber bands. The slide is placed in a sloping position, and 

 the fluids introduced by adding them in drops at the top side of the slip. 

 The excess fluid streams out on the lower side, or may be guided down 

 by strips of filter-paper. 



When the latter device is adopted, the rubber bands may, if desired, 

 be removed after the passage of the fixing fluid. The cotton acts as a 

 mechanical obstruction, the organisms being caught in the meshes. The 

 procedure is safe and rapid, and also convenient, as the preparations may 

 be inspected at any stage or stored up for future examination. 



Preparation of Metal Specimens for the Microscope.* — In pre- 

 paring specimens of metal, the principal object, says M. I. Cross, is to 

 obtain a perfectly level surface, free from all scratches and marks, with 

 the highest degree of polish. The surface of the sample is first care- 

 fully filed or ground. The marks made by this procedure are then 

 taken out with a very smooth file or with emery cloth, the coarseness 

 of the cloth being; gradually diminished until the finest grade is 

 reached. From this stage the polishing must be done on parchment 

 or chamois leather stretched very tightly on wood ; the leather being 

 sprinkled with fine crocus powder or rouge moistened with a little 

 water. At this stage the metal should be frequently examined under 

 the Microscope. This is easily done by clamping it in a metal-holder. 

 When the requisite degree of polish is attained the preparation is ready 

 to be etched, by which process the structure is further developed. 

 Etching is effected by various reagents, such as dilute mineral acids, 

 but best by infusion of liquorice root and tincture of iodine. The 

 method of applying the reagent is as follows : the specimen is either 

 coated with some protective varnish, — leaving the surface to be acted 

 on free, — and immersing the whole in a bath ; or a few drops may be 

 applied to the surface and then spread by means of a glass dipping rod. 

 The solution should be allowed to act, say, for 20 seconds ; the specimen 

 is then carefully washed in methylated spirit, the surface being gently 

 rubbed with the little finger, after which it is washed in water and 

 then dried with a soft piece of linen. If the etching be not satisfactory, 

 the process should be repeated. Owing to the advent of the metal- 

 holder it is no longer necessary to fix the piece of metal to a glass 

 slide, as this apparatus is fitted with jaws which grip the preparation 

 tightly and allow it to be set in any plane. 



(3) Cutting-, including- Imbedding- and Microtomes. 

 Born and Peter's Orientation Plate.|— This appliance, named after 

 its inventors, is made by Messrs. Zeiss, and is designed to mark the 

 position of objects contained in hardening paraffin, aud at the same 

 time to impress so-called lines of direction upon the paraffin blocks. 

 For the latter purpose one side of the plate has several parallel grooves, 

 of equal width and with sharp edges, cut into it. A pair of glass set- 

 squares is generally supplied with the instrument. 



* Knowledge, xxv. (1902) pp. 189-90. 



t Catalogue, English edition, 1902, p. 114. 



October loth, 190. 



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