54 PREPARATION AND MOUNTING 



means of a thick varnish and camel-hair pencil, to form a 

 ring of the desired depth ; but should the varnish not be of 

 sufficient substance to give such " walls " at once, the first 

 application may be allowed to dry, and a second made upon 

 it. A number of these may be prepared at the same time, 

 and laid by for use. When liquids are used (see Chapter V.)> 

 Dr. Carpenter recommends gold-size as a good varnish 

 for the purpose, and this may be used in dry mountings 

 also. I have used the asphaltum and india-rubber (men- 

 tioned in Chapter II.), and found it to be everything I could 

 wish. The cells, however, must be thoroughly dry, and when 

 they will bear the heat they should be baked for an hour at 

 least in a tolerably cool oven, by which treatment the latter 

 becomes an excellent medium. All dry objects which will 

 not bear pressure must be firmly fastened to the slide, 

 otherwise the necessary movements often injure them, 

 by destroying the fine hairs, &c. For this purpose thin 

 varnishes are often used, and will serve well enough for 

 large objects, but many smaller ones are lost by adopting 

 this plan, as for a time, which may be deemed long enough 

 to harden the varnish, they exhibit no defect, but in a while 

 a " wall " of the plastic gum gathers around them, which 

 refracts the light, and thus leads the student to false 

 conclusions. In all finer work, where it is necessary to 

 use any method of fixing them to the slide, a solution of 

 common gum, with the addition of a few drops of glycerine 

 (Chapter II.), will be found to serve the purpose perfectly. 

 It must, however, be carefully filtered through blotting- 

 paper, otherwise the minute particles in the solution interfere 

 with the object, giving the slide a dusty appearance when, 

 tinder the microscope. 



When mounting an object in any of these cells, the glass 

 must be thoroughly cleaned, which may be done with a 

 cambric handkerchief, after the washing mentioned in Chap- 

 ter II. If the object be large, the point of a fine camel-hair 

 pencil should be dipped into the gum solution, and a minute 

 quantity of the liquid deposited in the cell where the object 



