12 PRACTICAL BOTANY 



fluids which will mix. Students are warned against too readily 

 accepting negative evidence as the result of observations by 

 irrigation : the reagent may frequently pass under the cover- slip 

 without permeating the object, or the edges only of a section 

 may be affected : in order to insure the object being bathed by 

 the reagent, it is well to raise the cover-glass gently with a 

 needle, or even to raise the section itself slightly with the point 

 of a needle. 



8. Never use more than one cover-slip on a single slide, 

 though several objects may, if small enough, be covered by 

 one slip : the cover-slip should be as nearly as possible in the 

 middle of the slide. 



9. Pressure should never be laid on the cover-slip, except in 

 certain special cases : a bad section will not be improved by 

 being squeezed flat, while a good section may be easily ren- 

 dered worthless by such treatment. 



10. Before putting a slide aside for subsequent observation 

 be sure that the medium used is one which will not evaporate : 

 alcohol, water, and solutions in water, such as iodine solution, 

 aniline sulphate, salt solution, are all liable to evaporation, 

 while chlor-zinc-iodine and glycerine are not. 



11. Before putting a slide aside be careful to label it, writing 

 at once on an adhesive label the name of the plant, part, 

 direction of section, and medium in which it is mounted. 



B. Adjustment of the Microscope for Work. 



Before beginning work it should be ascertained that the 

 microscope is in good working order : if it be a stand without 

 rackwork, see that the stage of the microscope is clean, and 

 that the tube moves easily and smoothly ; if it does not, take 

 out the tube, and rub it with a clean cloth ; if still stiff, apply a. 

 very little mineral oil or vaseline, and rub with a clean cloth 

 till there is no appearance of oil on the tube : it should then 

 work freely. Nothing causes a greater strain on a microscope 

 than neglect of this simple point. 



Next see that the lenses are clean ; to this end first dust the 

 mirror, and adjust it so as to reflect light through the instrument ; 



