CUTTING VEGETABLE SECTIONS. 189 



subsequent cuttings. After every cut, the necessary thick- 

 ness must be sent up by means of the screw below, 

 and if the cutting has been done properly, i. e. by a firm 

 diagonal push from point to handle of the knife, keeping 

 this also back and edge at the same time upon the cutting 

 plate, the section will finally look like the illustration in 

 Fig. 98. 



During the cutting operation the knife or razor should 

 be kept flooded with dilute glycerine or weak alcohol, and 

 the section may be easily liberated from the knife by 

 gently shaking in a basin of water. The glycerine and 

 gum surrounding the sections dissolves after a few minutes, 

 allowing them to fall to the bottom, while the naphthalin 

 mixture floats on the surface of the water. 



Most vegetable substances can be cut in this way, leaves 

 and stems among the rest ; but some will require prepara- 

 tion before inserting into the microtome. Those which are 

 too hard must be soaked in water until soft enough to cut, 

 or even immersed in boiling water if necessary. Excess of 

 water may be abstracted by soaking in alcohol (methylated 

 spirit) for a time, or by partial desiccation. Whenever it 

 is required to eliminate resinous substances methylated 

 spirit will perform the operation, but it must not be for- 

 gotten that alcohol extracts the colouring matters from 

 most vegetable substances. 



Very good sections of certain woods can be made with 

 an ordinary smoothing plane, such as is used by joiners. 

 Deal may be cut in very thin ribands by this means, and 

 when mounted dry, constitutes Dr. Carpenter's test for the 

 colour correction of objectives. 



A word of. advice to the beginner may here be neces- 

 sary : discard all sections not showing the leading features 

 intended ; they are worthless from a scientific point of 

 view, yet it is strange how many there are in existence. 



