HOW TO MOukx SPECIMENS 43 



While melting the jelly, inchne the slide in order to allow the 

 melted glycerine jelly to flow gradually over the specimen, thus 

 replacing the air contained in the cells and intercellular spaces. 

 Finish the mounting as directed above, but under no conditions 

 should you stir the glycerine jelly with the section. 



If specimens, after having been embedded in paraffin or 

 collodion, are cut, cleared, stained, and dehydrated, they are 

 usually mounted in Canada balsam. A small drop of this sub- 

 stance, which may be obtained in collapsible tubes, is placed 

 at one side of the specimen. While inclining the slide, gently 

 heat until the Canada balsam covers the specimen. Secure a 

 cover glass by the aid of phers, pass it through the flame three 

 times, and lower it slowly while holding it in an inclined position. 

 Press gently on the cover glass with the needle-handle, and keep 

 in a horizontal position for twenty-four hours, then place directly 

 in a slide box or cabinet, since no sealing is required. 



Glycerine is sometimes used to make permanent mounts, but 

 it is unsatisfactory, because the cover glass is easily removed 

 and the specimen spoiled or lost, unless ringed — a procedure 

 which is not easily accomplished. If the specimen is to be 

 mounted in glycerine, it must first be placed in a mixture of 

 alcohol, glycerine, and water, and then transferred to glycerine. 

 Lactic acid is another permanent liquid-mounting medium, 

 which is unsatisfactory in the same way as glycerine, but like 

 glycerine, there are certain special cases where it is desirable 

 to use it. When this is used, the slides should be kept in a 

 horizontal position, unless ringed. 



COVER GLASSES 



Great care should be used in the selection of cover glasses, 

 however, not only as regards their shape but as to their thickness. 

 The standard tube length of the different manufacturers makes 

 an allowance of a definite thickness for cover glasses. It is 

 necessary, therefore, to use cover glasses made by the manu- 

 facturer of the microscope in use. 



Cover glasses are either square or round. Of each there are 

 four different thicknesses and two different sizes. The standard 

 thicknesses are: 



