96 Moderjn Microscopy 



fitted on top of them a removable cap with a very small 

 pin-hole. This pin-hole should be focused with, say, an 

 inch or a J-inch objective, and the condenser centred by 

 it ; the cap should then be removed from the condenser for 

 working. Condensers not provided with this cap can, as a 

 rule, be centred by using a diaphragm having a very small 

 aperture at the back of the lenses, and focusing the aerial 

 image of it with a J -inch objective ; but the easiest way of 

 centring is to make a very small spot in the middle of the 

 top of the lens with a pen and ink ; centre by this spot, 

 and wipe it off. It will not make any difference to the 

 performance of the condenser, and will ensure accuracy and 

 save time. Having centred the condenser, it should be 

 racked up until it touches the under- side of the slide, the 

 objective being made to touch the cover-glass of the object 

 on the upper side ; see that the diaphragm of the condenser 

 is open, reflect the light with the mirror, and thus illuminate 

 the field ; then rack the microscope body upwards until 

 the object comes into view. If it is found that there is too 



bright a flood of light, the aperture of 

 " ^ tl ! condenser must be decreased a 



] iMe by using a smaller diaphragm. 



I L ,ving focused the object on the 



1 per side with the objective, it will 



/ I 1 necessary to focus the condenser. 



1 ck this downwards from the object 

 s^ ^ -y slightly until the image of the 



'■"'"": lamp-flame is seen in the centre of 



Fig. 31.— Image OF ,, \. ,j .,, . j , . 



Lamp-flame. toe held, the remamder being com- 



paratively dark, as in Fig. 31. If 

 now it be desired to have the whole field equally brilliant a 

 bull's-eye stand condenser may be interposed between the 

 lamp and the mirror, the plane side of the bull's-eye being 

 towards the lamp ; or the burner of the lamp may be 

 turned round till the flat of the wick is towards the mirror. 

 Where high powers are to be used, the object to be 

 examined may with advantage be set upon the stage and 



