THE MICROSCOPE AND ITS PARTS. 43 



will still be an inch long, but one-eighth of an inch 

 broad. Take one of the small brass hinges to be had for 

 a cent, solder one end to the hollow handle and the other 

 to the narrow doubled strip; into this narrow piece place 

 a thin glass square, the thinner the better, and the instru- 

 ment is done. To use it, turn the microscope horizontal, 

 have a faint light on the object and a strong one on the 

 paper, bend the strip of brass around the upper part of 

 the eye-piece so it will not slip, the hollow handle and 

 hinge being directed towards the table, and move the 

 hinge until the thin cover is placed obliquely in front 

 of the eye-glass of the eye-piece. Look down through 

 the glass square towards the paper on the table, and the 

 image of the object on the stage will seem to be thrown 

 on the white surface, where it can be traced with a pen- 

 cil. The image is really reflected from the surface of 

 the thin square, and the pencil is seen through it, but the 

 eye unconsciously combines them so that both are seen 

 together. The secret of success here is a faint light on 

 the object, a strong one on the paper, and a thin glass 

 square. A long, sharp pencil-point is also an advantage. 

 A micrometer is for measuring objects under the mi- 

 croscope. It is made by ruling a number of short lines 

 on glass, the spaces between the lines varying from j^-g- 

 to 10 1 6& - inch or less. Micrometers are said to have been 

 ruled with one million lines to the inch, but no human 

 eye using the best and highest power objectives has ever 

 seen them. All micrometers are ruled by a machine 



made for the purpose. 

 3* 



