HOW TO PREPARE AND EXAMINE SECTIONS 63 



to cover the object. For examination under the microscope, 

 the section must be covered with a cover-glass ; these can be 

 obtained of different sizes, the one in general use being ^ of an 

 inch in diameter. 



If the plant from which the sections have to be prepared has 

 been kept in spirits, they must be mounted either in alcohol or 

 glycerine. The section must never be allowed to dry ; if it 

 does, air fills up the cells, and the air bubbles, as they are 

 called, make the specimen appear very dark. 



(1) Do not begin to cut sections until you are quite certain what 

 kind you require. If the sections are to be viewed by the compound 

 microscope, keep the razor or knife wet by dipping it before each cut 

 into a glass of water for fresh specimens, and in spirits for materials 

 which have been preserved in alcohol. 



(2) Keep the microscope perfectly clean, and be careful that no 

 mounting fluid finds its way on to the stage, lens, or any parts of the 

 instrument. 



(3) In mounting the specimen, only just sufficient of the mounting 

 medium must be used to cover the object. When the cover-glass is put 

 on, it must on no account be allowed to drop ; one edge must first 

 touch the mounting fluid and be slowly lowered into position, so as to 

 spread out the medium and drive out the air. This can be done by 

 using a needle or pin to support the cover-glass, and with the thumb 

 and finger of the left hand guide it into position as the needle or pin is 

 slowly withdrawn. 



(4) Always keep the section wet so as to avoid air-bubbles. If air- 

 bubbles are found in a specimen, they may be removed by gently 

 warming the slide by placing it over ajar containing hot -water. 



(5) If the specimen is intended for future use be careful to label it, 

 and write on this label the name of plant, portion of plant, direction of 

 section, mounting fluid, and date. 



EXPT. 51. Take a glass slip, and with a dipping rod place a single 

 drop of water in the centre. Now place a cover-glass over the drop of 

 water. This can be done by using a needle or pin to support the cover- 

 glass, and with the thumb and finger of the left hand guide the cover- 

 glass into position, at the same time slowly withdrawing the needle or 

 pin. After a few attempts the pupil will be able to spread out the drop 

 so as to fill the entire space beneath the cover-glass. 



The Structure and use of a Hand-Lens. A lens is a 

 transparent substance, usually formed of glass, and is shaped so 

 as to change the direction of the rays of light which pass through 

 it. A lens appears to magnify or diminish the size of objects 

 seen through it. A hand-lens is a piece of glass, suitably 

 mounted, which possesses the property of magnifying objects. 



