522 THE MICROSCOPE. 







piece consists of a larger lens of greater focal length 

 (a few centimetres). As the objects to be observed 

 vary in thickness, the tube Tc k must be capable of being 

 raised and lowered. For this purpose it slides with 

 gentle friction in the tube h h, and can thus be approxi- 

 mately placed in the proper position by the hand. In 

 order to give to it a finer motion, the tube h h is fixed by 

 the horizontal arm a to the cylinder r, which may be 

 raised or lowered through a small space by turning the 

 milled head m of a fine screw either to the right or to 

 the left, Fig. 284 explains the mode of action of the 

 instrument. The arrow a b represents the object, c the 

 object lens, of which/! is one focus, while the other is 

 at the opposite side near a b'. Since the distance of the 

 object from the lens c is less than twice the focal length, 

 the lens produces a strongly magnified image a^ b^ the 

 position of which is found exactly as in fig. 272, by 

 drawing two rays from each extremity of the object a b, 

 one ray parallel to the axis, the other through the 

 centre of the lens. This image is seen through the 

 lens de of the eye-piece, which has its focus at/ 2 . For 

 this lens the image a x b is now the luminous object, and 

 being at a less distance from the lens than the focal 

 length of the latter, a magnified virtual image a 2 b 2 is 

 produced, which may be found in the same manner as 

 that in fig. 273. It is the image which is seen by an 

 eye looking vertically downwards into the microscope. 

 The images, a^ b l and a. 2 b. 2 are inverted with respect 

 to the object. The eye-piece of a microscope acts 

 exactly like a common inagnifying-glass. 



The construction of a good microscope can only be undertaken by 

 a skilled optician ; and as the grinding and adjustment of the lenses 



