A SIMPLE DESCRIPTION OF THE MICROSCOPE 23 



the image is a fine bundle thrown to the edge of the object glass, 

 so that the object glass acts as if it had only a pinhole aperture 

 at one side, and is consequently no better for depicting detail than 

 the early pinhole lenses which were made before the modern 

 achromatic microscope was discovered. 



This diagram also shows how the direction of the light can be 

 immediately recognised by focussing the microscope. The only 

 light producing the primary picture is shown in the left-hand 

 diagram of Fig. 10. It is on the right or the left of the axis, 

 according to whether it is above or below the true focus. There- 

 fore, by putting the object in and out of focus with the focussing 

 adjustment, the direction of the light can be observed. When 

 the light which illuminates the object is oblique instead of being 

 truly central, the object wiU not only become indistinct on either 

 side of the focus, but will appear to move from side to side ; whereas 

 if the light is truly central, the object will become less distinct 

 on either side of the focus, but will not alter its position. 



The mirror can always be adjusted until the object remains || 

 stationary as the microscope is being focussed, and the centring I* 

 of the light is thus assured. 



Below the stage of the microscope an iris diaphragm (K, Fig. 1) ~ 

 is fitted, and if this is shut down to a small aperture the light will 

 not pass through the microscope at all if the light is very far away 

 from the axis, though this is not in itself sufficient to make the 

 final adjustment. 



The nature of the illumination may be varied according to Nature ot 

 whether it is parallel, divergent, or convergent. If the flat side 

 of the mirror be used and the , 



source of light is at a consider- ™,;;: (ji| ';!!i;;:'.'',' 



able distance, a beam of nearly jB ^,'J 



paraUel light is obtained {a, Jili;': /S 



Fig. 11). If the source of illu- ^^~-- ^""'' 

 mination is very close, a diver- 

 gent beam is obtained (c, Fig. 11). 



If the concave mirror is used, it o 6 c 



will be a slightly convergent Fio. 11. — Mirror refleotingparallel, 

 beam (5, Fig. 11). By means of convergent, or divergent light. 

 a substage condenser (J, Fig. 1) 



with an iris diaphragm below it — described later — the light 

 can be rendered still more convergent and can be regulated 

 with accuracy. As the light which enters the condenser at its 

 margin emerges as the outer portion of the cone, the effect of 

 reducing the aperture of the diaphragm of the condenser is not 

 only to reduce the amount of illumination, but to alter its 

 character by reducing the size of the cone of emergent light. 

 Thus with a very small aperture an almost parallel beam of light 

 can be obtained, and by opening the iris diaphragm a more and 

 more highly convergent cone of light may be used (see p. 27). 



