PRACTICAL WORK 33 



we are far from understanding the whole secret of life, 

 we do know that many of the manifestations of life 

 depend upon the chemical nature and physical proper- 

 ties of protoplasm and of the substances derived from 

 it. The next two chapters will therefore be devoted to 

 these subjects. 



PRACTICAL WORK. 



A. Use of the Microscope. 



(i) Examine the microscope, especially the mirror, substage 

 diaphragm, low and high power objectives, eyepieces, coarse and 

 fine focussing adjustments. [The demonstrator should show the 

 students how to handle a microscope, explain the outlines of its 

 working, and give the necessary cautions.] 



(2) Put a small drop of water on the middle of a slide, and 

 place in it a few grains of sand. Cover with a coverslip so as 

 to include in the water some bubbles of air. 



Examine first with the low power and then with the high 

 power 



{a) a sand grain, noting the shape, colour and translucency ; 

 (b) an air bubble, noting its appearance in different focal 

 planes. 



(3) Mount a single leaf from a moss plant in a drop of 

 water. Cover with a coverslip so as not to include bubbles 

 of air. 



Examine first with the low and then with the high power. 

 Note that the leaf consists of a single layer of box-like compart- 

 ments (cells), with transparent walls, through which can be seen 

 the green chloroplasts containing the pigment chlorophyll. The 

 cells of the moss leaf are a good type of green plant cell. 



(4) Determine the apparent length in fi^ in the field of the 

 microscope (with fixed tube length) of a division of the eyepiece 

 micrometer scale, (a) with the high and (6) with the low power 

 objective, by determining the correspondence of the eyepiece 

 scale with the divisions of the measured scale engraved on the 

 micrometer slide. Make a note of these valines for future use. 

 Now measure the length and breadth of a cell of the moss leaf and 

 the diameter of a chloroplast under the high power. 



' /< = 'oor mm. 



