INTRODUCTION. 5 



turning the eyepiece in the tube while looking through 

 it. In like manner by partly unscrewing the eye-lens and 

 turning it, one may discover whether the eye-lens or 

 field-lens is dirty. If the front lens be dirty, it will be 

 shown by a dimness and want of definition of the out- 

 lines of objects, thus affecting the whole field of view. 



In focusing, use first the low- power objective. By using 

 the rack and pinion adjustment lower the objective well 

 down toward the cover-slip. Then while looking into 

 the eyepiece, after having made sure that the best light 

 is reflected by the mirror up through the diaphragm in 

 the stage, slowly raise the objective until the object comes 

 into view. When it is desired to use the high- power 

 objective, carefully lower it until it is almost in contact 

 with the cover-slip. Then while looking into the eyepiece 

 focus upward until the object is seen clearly. 



Some nosepieces are so constructed that when the 

 low power is in focus the high power may be turned 

 directly into focus without changing the elevation of the 

 objectives. In no case should one focus downward 

 while looking into the eyepiece unless the object is al- 

 ready in view. Failure to observe this caution may 

 result in forcing an objective down upon the slide. Also, 

 before placing or removing a slide the low- power objec- 

 tive should be turned into position for use. 



Before leaving the laboratory at the close of a labora- 

 tory period, make sure that all the instruments are thor- 

 oughly clean and dry and in their proper places. 



2. Illustrative material. In studying plants in the 

 laboratory it must always be kept in mind that one should 

 find out as much as he can about the structures and their 



