524 THE MICROSCOPE. 



brass. The paper is slightly oiled, so as to render it translucent, 

 and in the brass plate six small hole3 are made, so as to form a 



cross ".' The holes are not punched right through, but the 



brass plate is laid on a piece of lead ; the punch receives only a 

 slight blow with the hammer so as to produce a depression, and the 

 file is then applied to the elevations on the opposite side of the brass 

 plate until the metal is filed quite thin, and a hole may be made 

 through the brass by pushing a middle-sized needle through it. 

 The brass plate must be pressed into the incision just sufficiently to 

 bring the cross into a line with the middle of the lenses, when the 

 cork ft, which carries the plate, is placed upon the board in the 

 manner shown in the figure. Before the brass plate is put in posi- 

 tion, the cork a with the oiled paper is placed about 4 cm from the 

 lens c, and looking at it through the lens the cork is moved to and 

 fro until the paper is seen as sharply defined as possible. A flame 

 is then placed at the other end of the board, exactly in a line with 

 the middle of both lenses ; and finally the cork with the brass plate 

 set upon the board, about I7 mm '5 from the lens d, and moved to and 

 fro until a distinct inverted image of the cross appears upon the 

 oiled paper. With the distances assumed the image on the paper 

 is magnified about four times : the lens c magnifies this image five 

 times, and hence the image seen through c appears 4 x 5 = 20 

 times larger than the object. The paper screen serves for showing 

 that a real image is produced by the object lens, denoted by a l l l in 

 fig. 284. If the paper is removed, the magnified image of the cross 

 is seen still better defined by the eye looking through c. This final 

 image, a 2 & 2 i 11 fig- 284, is virtual, and can therefore not be shown 

 on a screen like the real image formed by the object lens. 



The experiment must be made in a darkened room, otherwise 

 the real image on the oiled paper will not be distinctly seen. The 

 lenses of a common microscope are usually fixed in their tubes, 

 which are blackened inside. All light which does not proceed from 

 the object itself is thus shut off. In our arrangement the tube 

 is left out in order to exhibit the formation of the image on the paper. 

 If the latter is removed, we may see images of objects which do not 

 even emit as much light as the cross. Thus by holding a piece of 

 fabric in the place of the cross, we may see the threads pretty 

 strongly magnified. A magnifying power of 20 is, however, very 

 small for a microscope. The magnifying power is increased by 

 using object lenses of much shorter focal length, and good micro- 

 scopes have generally a number of object glasses which may be 

 screwed into the end of the tube, and by which the magnifying power 

 may be made to vary between 30 and 500 times and even more. 



