4 INTRODUCTION. 



III. HOW TO USE THE LABORATORY. 



i. Hand-lenses and compound microscope. All that 

 can be determined concerning the specimen in hand 

 without magnification should be done before use is made 

 of magnifying-glasses. The use of hand or dissect- 

 ing lenses and the compound microscope should never 

 be considered an end in itself, but merely a means of 

 obtaining a better idea of plants than is possible without 

 these things. 



A prime requisite in the use of any optical instrument 

 is cleanliness; dirty lenses frequently defeat the very 

 object of their use, namely, clearer vision. Before begin- 

 ning to work with any lens, see that it is perfectly clean. 

 When a lens needs cleaning a camel's-hair brush may 

 first be used to brush away any particles of dust. Then 

 wipe gently with a piece of lens-paper or unstarched 

 linen or cotton, breathing first upon the lens to moisten 

 the dirt. Too great care cannot be taken to avoid 

 scratching the polished surface of the lens; hence in 

 wiping it the least possible effective pressure should be 

 used. If properly handled after once thoroughly cleaned, 

 lenses will seldom need any cleaning except brushing. 

 One should avoid touching the lens with the fingers, 

 since the oil from the skin adheres to the glass and 

 temporarily impairs its usefulness. Such spots may be re- 

 moved by wiping with linen slightly moistened with alco- 

 hol. In using the compound microscope the front only 

 of the objectives and both surfaces of both lenses of the 

 eyepieces need cleaning. If the eyepiece be dirty, there 

 will be specks in the field of view when there is no object 

 upon the stage. These can be made more apparent by 



