July 26, 1883] 



NA TURE 



301 



of hot water, and the slide should be warmed also if this 

 can be done without injury to the object. The varnish 

 should then be applied with a camel-hair brush. It dries 

 in a few hours at the ordinary temperature, or in a few 

 minutes at the temperature of a cool oven, but it has not 

 the tenacity of gold size, and is liable to become brittle 

 with age. To keep the cover in place during the harden- 

 ing of the cement, spring clips will be required. One 

 very useful form can be made by bending a piece of brass 

 wire into the shape shown in Fig. I, and fixing it by 

 means of glue into the end of a piece of cedar (end of 

 cigar box) a little larger than the slide. 



Fig. i. 



When the object is of considerable thickness or when 

 it would be injured by the pressure of the cover glass, a 

 wall or cell of some kind must be raised round it. In 

 general a very shallow cell made by drawing a ring of 

 gold size or asphalte on the slide is sufficient, and a stock 

 of these cement cells of various sizes should be always 

 ready for use. For their manufacture and for finishing 

 the slides a turntable should be provided. This in its 

 simplest (and in the writer's opinion its best) form con- 

 sists of a heavy brass disk 3^ inches in diameter, capable 

 of rotation in a horizontal plane on a central steel pin. 

 The slide is held in a central position on thi s table by 

 two spring clips. Then on whirling the table round and 

 applying to the slide a brush charged with varnish, a neat 

 circle will be struck out. 



When cells of greater depth are required, solid rings 

 must be cemented to the slide. 



For the performance of such dissections as are neces- 

 sary, the mounter will require two or three small scalpels, 

 one or two razors, a pair of small scissors with sharp 

 points, and two pairs of forceps, one large, with its points 

 roughened where they meet, and one small and slender, 

 with smooth points. Small camel-hair brushes and com- 

 mon sewing-needles fixed in cedar handles like those used 

 for the brushes are indispensable. 



Pipettes of various sizes are useful for transferring 

 small quantities of liquids or catching small aquatic ani- 

 mals. They are easily made from pieces of glass tube of 

 various sizes, some being left widely open and others 

 drawn off to a point at one end, which may be left 

 straight or bent at a small angle. The most useful form 

 of pipette is made by tying a piece of sheet indiarubber 

 a:ross the bell of a very small thistle funnel, the stem of 

 which may be either left widely open or drawn to a point 

 as with other pipettes. Pressure with a finger on the 

 indiarubber will displace a quantity of air, and when the 

 open end is placed under water and the pressure removed 

 a quantity of the liquid will be drawn up and can be 

 removed and delivered drop by drop or in a rapid stream. 

 If (the indiarubber being pressed down) the open end of 

 the tube be brought near any small animal in the water 

 and the pressure suddenly relieved, there will be such a 

 rush of water into the tube that the strongest swimmer 

 can be easily captured. 



Two or three section-lifters of various sizes and a 

 dozen watch-glasses for holding staining solutions will 

 complete the list. 



The objects of mounting are twofold : (1) to render 

 visible structures that could not be seen without such 

 preparation, and (2) to preserve the bodies so prepared as 

 permanent objects for future study. 



Various fluid media are employed for the preservation 

 of objects, and much of the mounter's success in his art 

 depends upon a knowledge of the medium most suitable 

 for each particular object. In Figs. 2 and 3 an attempt 



has been made to show how largely the visibility or invisi- 

 bility of particular structures is determined by the nature 

 of the medium in which they are mounted. 



Both of these figures represent longitudinal sections of 

 the stem of the spruce fir cut from the same shaving of 

 a deal plank, the only difference being that the former 

 (Fig. 2) was mounted in air, and the latter, after staining, 

 was mounted in balsam. In the former case the bordered 

 pits in the wood cells are perfectly shown, but the boun- 

 daries of the cells themselves and the medullary rays are 

 indistinct and confused, while in the latter case the wood 

 cells and medullary rays are clearly defined, but the 

 penetration of the highly refractive balsam which has 



Fig. 2. — Longitudinal section of stem of spruce pine mounted dry, \' 

 objective. 



affected this change has reduced internal reflection so 

 far, and rendered the whole section so transparent, that 

 the pits have become almost invisible. 1 



The same truth was forcibly brought home to the writer 

 a few years ago in cutting some sections of fossil coni- 

 ferous wood (siliceous), which during the latter stages of 

 grinding down displayed the characteristic glandular cells, 

 &c, admirably, but when mounted in balsam became almost 

 perfectly invisible. They were too opaque to be mounted 

 dry, and the only liquid in which they were well displayed 

 was distilled water. The sections mounted in balsam 

 were by no means spoiled though, for the transparency 

 which obliterated all structure when viewed by ordinary 

 light rendered them peculiarly suitable for examination 



Fi'j. j — Longitudinal section of stem of spruce pine mounted in balsam, \" 

 objective. 



by polarised light, and when so viewed all their structure 

 returned and they became most beautiful objects. 



It would be impossible in these articles to describe all 

 the media employed in mounting microscopic objects, and 

 all that will be attempted is to give instructions for 

 mounting objects dry (that is, in air), in balsam, and in 

 glycerine jelly. 



The dry method is employed for such objects as are 

 unaffected' by air, and are either intended to be viewed as 

 opaque objects by reflected light, or are sufficiently trans- 

 parent without previous preparation to be examined by 

 transmitted light. The object of this method is, in fact, 

 simply to afford mechanical support to the object, and to 

 protect it from dust and moisture. 



It is necessary that the objects should be perfectly dry 



1 In Fig. 3 the pits are shown much too plainly. 



