190 THE MICROSCOPE 



may readily be cut in the microtome. The sections should be 

 mounted unstained in balsam, and though not usually regarded 

 as polariscopic objects, nevertheless, when examined with the 

 selenite, yield very fine colours. 



Wood. — Shavings of extreme thinness may be cut from large 

 pieces or blocks of timber by means of a very sharp plane. In 

 this way excellent sections may be procured of most of the 

 common woods, as oak, mahogany, pine, etc. Where, however, 

 the material to be operated upon takes the form of stems, roots, 

 etc., of no great thickness, they should, after having been reduced 

 to suitable consistence, be imbedded in paraffin and cut in the 

 microtome. Before imbedding, it must not be forgotten to 

 immerse the wood to be cut in weak gum-water, this precaution 

 being of great importance, especially in the case of stems, etc., 

 of which the bark is at all rough and sinuous. If the sections are 

 to be mounted unstained, they are usually put up in weak spirit 

 (mixture of spirits of wine i part, distilled water 3 parts). For 

 mounting, proceed as follows : — A ring of gold-size must, by means 

 of the turn-table, be drawn in the centre of the slide, and put away 

 in a warm place for several days (the longer the better), in order 

 that the ring may become perfectly dry and hard. The section, 

 with a drop or two of the mixture, is to be put in the centre of 

 the ring, and a cover-glass of requisite size having been cleaned, 

 this must have a thin ring of gold-size applied round its margin. 

 The cover is now placed in position, and gently pressed down, 

 the pressure apparatus being employed, if necessary, to prevent it 

 from moving. In about twenty-four hours another layer of the 

 varnish should be applied, and the slide afterwards finished in the 

 manner already described. A very general method also of dealing 

 with this class of objects is to mount them dry. This plan cannot, 

 however, be recommended, as the sections always present a 

 disagreeable black or blurred appearance. To avoid this, we 

 may have recourse to Canada balsam, but the ordinary method of 

 employing it must be slightly modified, a drop of chloroform being 

 substituted for the clove oil, otherwise this latter agent would 

 cause the section to become so transparent as to render minute 

 details of structure difficult to recognise. 



