DEVICES FOR SEEING 299 



turn the cover glass over so the liquid glass or balsam will hang 

 from the under side, the lower surface of it in the shape of the 

 segment of a sphere. Allow this to stand until it hardens. A 

 lens of this sort may be used in making a microscope or for the 

 eyepiece of a telescope. The large-sized lenses made from watch 

 crystals are serviceable also as objectives for telescopes or magic 

 lanterns or as condensers for magic lanterns. Directions for 

 making a microscope, telescope, and magic lantern are given 

 in the Field and Laboratory Guide in Physical Nature-Study. The 

 principle of operation may be explained here. 



The microscope consists of two lenses mounted at the opposite 

 ends of a tube which is about i inch in diameter and several 

 inches long. One of these lenses, the one through which you 

 look, is the eyepiece; the other, which is brought close to the 

 object to be examined, is the objective. The object to be 

 examined is brought near enough to the front of the objective so 

 that an image is formed up in the tube of the instrument just below 

 the eyepiece. This image is then examined by the eyepiece, which 

 serves as a magnifying glass (Fig. 146) . Recalling our experiment 

 with the convex lens and the candle flame, it will be remembered 

 that when the flame was near the lens the image was relatively far 

 from the lens and larger than the object. The image formed below 

 the eyepiece is therefore enlarged, and when the eyepiece magnifies 

 it still more, one sees the object hundreds or even thousands 

 of times larger than it really is. There are some accessory 

 parts to the microscope (Fig. 147), which make it more con- 

 venient, but the lenses held by the tube are the essential things. 

 There is usually a heavy base on which the instrument stands, 

 and a pillar that carries the tube on a movable arm. This pillar 

 also bears the stage on which the object to be examined is placed, 

 and a mirror to throw light on the object to be examined. In 

 addition there is a coarse adjustment that moves the tube 

 rapidly up and down by means of rack and pinion, and a 

 fine adjustment that moves it very delicately. The objec- 

 tives, especially of a good microscope, are built of several lens 



