Laboratory Equipment. 373 



the middle, ring, and little fingers of the hand holding the lens should 

 rest, in a closed position, in the palm of the hand holding the object, as 

 shown in Fig. 197. It is to be remembered that the object, and not 

 the eye or the lens, should receive the best illumination ; for the object 

 becomes visible only by the light which is reflected from its surface 

 through the lens and into the eye. The best light is that which comes 

 over the shoulder or from one side, and the student, if facing a window, 

 should turn to one side when making observations. For some work 

 the dissecting stands are a great convenience, particularly when minute 

 structures require separation with the needles. In such a case the 

 object should lie upon the dissecting stage, and the lens should be 

 placed in the wire holder and adjusted to a proper height to give a sharp 

 image of the object ; then both hands will be free to manipulate the 

 specimen with the needles while it is viewed through the lens. In 

 using the needles, the hands should rest upon the shelves made for that 

 purpose at the ends of the block. 



240. Care of the Lens. — The lens must be kept clean and bright. 

 It will be noticed that if the fingers come in contact with its faces a 

 filmy spot is left which excludes much of the light when the lens is 

 again used. To remove such spots, breathe upon the glass and quickly 

 polish it with a clean soft cloth. The student should always see that 

 the lens is in good condition before using it. 



241. The Compound Microscope. — A compound microscope is not 

 an absolute necessity in a beginning course in botany, but it is very use- 

 fijl in gaining a clear comprehension of some primary facts of plant 

 structure and physiology with which the student should early become 

 acquainted. A good compound microscope with a very satisfactory out- 

 fit can now be purchased by schools for $25 or less, and there is no 

 reason why every school in which botany is taught should not be sup- 

 plied with at least one. The optical parts of a compound microscope, 

 which are most useful for a general study of tissues and cell contents, 

 are a i-inch eyepiece and a |-inch and a J-inch objective. The 

 objectives should be fastened to a double nosepiece in order that they 

 may quickly be shifted. 



242. Use of Compound Microscope. — When thin sections or very 

 minute objects, such as starch grains or cell contents in general, are 

 being studied, the object should be seen by transmitted light, — that is, 

 by light which is reflected from the mirror (0, Fig. 198), below the stage 

 («), through the object (/), and into the objective (k). The surfaces 



