374 



Introduction to Botany. 



of opaque objects are to be seen by light which is reflected directly 

 from their surfaces into the objective. The compound microscope dif- 

 fers from the simple lens essentially in 

 that the light, which passes from the 

 object into the objective, is brought to 

 a focus in the tube of the microscope, 

 at the level of the eyepiece diaphragm, 

 forming an enlarged real image (r), 

 which in turn is magnified by the eye- 

 piece (/). Thus, the eyepiece serves 

 as a simple lens and magnifies as at (7) 

 an enlarged image of the object. It 

 will be seen that it is of utmost impor- 

 tance that the lenses of eyepiece and 

 objectives should be kept perfectly 

 clean, otherwise the light will be too 

 much absorbed to produce a clear image. 

 In examining cell contents or thin sec- 

 tions of plant tissues with the com- 

 pound microscope, the objects should be 

 mounted in a drop of water or other 

 reagent, as the case may require, on a 

 clean glass slip, and covered with a 

 perfectly clean coverglass. 



243. Preparing Material for Obser- 

 vation. — In becoming acquainted with 

 the use of a compound microscope, starch 

 from a potato will do for examination. 

 Having washed a slip and coverglass and 

 polished them with a clean cloth, set 

 the coverglass on edge until wanted, 

 and lay the slip flat upon the table ; 

 then cut a potato in two and scrape off" 

 a very small portion of the pulp with the point of a penknife. Trans- 

 fer the material on the knife to a small drop of water placed at the center 

 of the glass slip, and cover with the coverglass. In doing this, grasp 

 the coverglass by its opposing edges between the thumb and forefinger, 

 and place its lower edge against the slip a little to the left (if the cover- 

 glass is held in the left hand) of the object ; then place the middle finger 



Fig. 198. 



Diagram of a Compound Micro- 

 scope, showing the manner of 

 the formation of the image. 

 See description in the text. 



