54 Modern Microscopy. 



this cap can, as a rule, be centred by means of their 

 diaphragms at the back, focussing the aerial image of it 

 when closed with a |-inch objective ; but the easiest way 

 to do it is to make a very small spot in the centre of the 

 top of the lens with ink, centre by this, and wipe it off. It 

 will not make any difference to the performance of the 

 condenser, and will ensure accuracy and save a lot of 

 trouble. 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 object on the 

 upper side ; see that the diaphragms of the condenser are 

 open, reflect the light with the mirror, when the field will 

 become illuminated ; then rack the microscope body upwards 

 until the object comes into view. It may be found that 

 there will be too great a flood of light. If so, the aperture 

 of the condenser must be decreased a little by means of 

 the diaphragm. Having focussed the object on the upper 

 side with the objective, it will be necessary to focus the 

 condenser. Eack this downwards from the object very 

 slightly until the image of the lamp-flame is seen in the 

 centre of the field, the remainder being comparatively dark, 

 as in Fig. 15. If now it be desired to have the whole field 

 equally brilliant, turn the burner of the lamp round till the 

 flat of the wick is towards the mirror. 



The next question is, What amount of light should be 

 admitted from the condenser in order 

 to see the object at its best? Mr. 

 Nelson has suggested and demon- 

 strated that the aperture of the con- 

 denser should be about three-quarters 

 that of the objective, and in order to 

 ascertain this it will be necessary to 

 remove the eyepiece from the micro- 



FIG. ISImage of SCO P e > and look at the back f the 

 Lamp-flame. object glass, opening the diaphragm 



of the condenser to its fullest extent. 

 Bearing in mind the size seen, gradually decrease, the size 



