OF MICROSCOPIC OBJECTS. 29 



the heat they should be baked for an hour at least in a tolerably 

 cool oven, by which treatment the latter becomes a first-rate 

 medium. All dry objects which will not bear pressure must be 

 firmly fastened to the slide, otherwise the necessary movements 

 very often injure them, by destroying the fine hairs, &c. For 

 this purpose thin varnishes are very 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 fix- 

 ing them to the slide, a solution of common gum, with the addi- 

 tion of a few drops of glycerine (Chapter I.), 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 appear- 

 ance when under the microscope. 



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

 be thoroughly cleaned, which may be done with a cambric hand- 

 kerchief, after the washing mentioned in Chapter I. 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 depo- 

 sited in the cell where the object is to be placed, but not to cover 

 a greater surface than the object will totally hide from sight. 

 This drop of gum must be allowed to dry, which will take a few 

 minutes. Breathe then upon it two or three times, holding the 

 slide not far from the mouth, which will render the surface adhe- 

 sive. Then draw a camel-hair pencil through the lips, so as to 

 moisten it slightly (when anything small will adhere to it quite 

 firmly enough), touch the object and place it upon the gum in 

 the desired position. This must be done immediately to ensure 

 perfect stability, otherwise the gum will become at least partially 

 dry and only retain the object imperfectly. 



When, however, the objects are so minute that it would be im- 

 possible to deposit atoms of gum small enough for each one to 

 cover, a different method of proceeding must be adopted. In this 

 ease a small portion of the same gum solution should be placed 



