376 



Introduction to Botany. 



micrometer screw. The image sliould soon appear, for the J-inch 

 objective is in focus when its front lens is very close to the cover- 

 glass. 



It will be noticed that there is a diaphragm (;«) beneath the stage 

 of the microscope, with openings of different sizes, or, in case of an iris 

 diaphragm, with a single opening which can be enlarged or contracted. 

 The larger openings give greater illumination, and the smaller, sharper 

 definition. It is a good rule to employ the smallest opening which per- 

 mits sufficient illumination with a given object and source of light. If 

 the diaphragm is of the revolving kind, care must be taken that the 

 opening employed is centrally placed, as indicated by the stop which 

 clicks into position. When scraped or powdered material has been 

 mounted in a drop of fluid under a coverglass, it may be spread out in 

 a thin even layer by rubbing the coverglass around with gentle pres- 

 sure, by means of the finger covered with a clean cloth. 



Fig. 199. 



Positions of the razor with reference to the stem in cutting : L, a cross section ; il/, 

 a longitudinal tangential section ; N^ 2l longitudinal radial section. 



245. Cutting Thin Sections. — Thin sections need to be cut from roots, 

 stems, leaves, etc., in order to study their cellular structure. In the case 

 of stems and roots, cross sections, and longitudinal sections through 

 the center, called longitudinal radial sections, are the most instructive, 

 but it is often desirable to prepare longitudinal sections outside the 

 center, termed tangential sections (Fig. 199). In the case of leaves, 

 cross sections of the blade, at any desired place, answer the usual 

 purposes. 



A good, half hollow-ground razor — that is, one with a thick blade 

 ground slightly concave on both sides — is more effectual in section-cut- 

 ting than the plano-concave razors sold by the dealers in microscope 

 supplies. Material which is fresh may be sectioned at once, but dry 



