Lecture /. 3 



with the object, but on the other hand it may cause inconvenience 

 if it adheres to its surface. 



The bubble has become enclosed because the outer surface of 

 the piece of skin acts as if it were greasy and is not wetted by the 

 water which is slightly raised up round the object. When the 

 cover-glass is put on, the air in the hollow immediately over the 

 piece of skin is trapped. If before the cover-glass is applied the 

 piece of skin is wetted by stroking it with a camel's hair brush or 

 a needle, and the cover-glass then let down slightly obliquely so 

 that one edge touches the water first, no air will be enclosed in 

 the preparation. 



The preparation should now be inspected to see that there is 

 no water on the under side of the slide, and no water or dust on 

 the upper side of the cover-glass. Water on the slide may be 

 carefully dried off, but if the cover-glass is not perfectly clean it 

 must be taken off again (raising it with a needle or point of a for- 

 ceps), carefully polished and replaced. 



The preparation thus made is now ready for examination. 

 Place it on the stage of the microscope. I will assume that the 

 microscope is provided with two objectives (powers): one (the 

 low power) focussing about i cm., and the other (the high power) 

 focussing i or 2 mm. above the object. Set the front lens of the 

 low power about i cm. above the cover-glass by means of the 

 coarse adjustment. Look into the eye-piece and turn the mirror 

 until the eye-piece appears filled with light. Close the diaphragm 

 below the condenser till its opening is about \ of the total 

 aperture. Turn the coarse adjustment until the object comes 

 into focus. Now turn the fine adjustment until you find the 

 position in which the object appears with greatest distinctness. 

 It is usually best to use the flat surface of the mirror when the 

 condenser is in position, but sometimes with this arrangement ex- 

 traneous objects such as the woodwork of windows are focussed 

 on the object and become inconvenient. This inconvenience may 

 be removed by using the concave surface of the mirror, or by 

 altering the level of the condenser. 



When sharp focus has been obtained you will see that the piece 

 of leaf-skin is divided up into small and fairly uniformly sized 

 areas. They are nearly rectilinear in outline, and fit closely 

 against one another like tiles in a pavement. These areas cor- 

 respond to the outlines of cells the units of which the skin and 

 all other tissues are built. A darkish rounded body may be seen 

 in most of these cells. It is called the nucleus. While mak- 

 ing these observations the inclination of the mirror should be 



