CRITICAL ILLUMINATION 119 



object ; when this is as sharp as possible the condenser 

 is said to be focussed. If the image of the lamp-flame is 

 not quite central a slight movement of the mirror will 

 make it so, 



5. The Appearance of the Back Lens of the Objective. 

 The back lens of the objective should be examined by 

 removing the eyepiece ; or its image, formed by the 

 eyepiece, called the " Ramsden circle," may be examined 

 by a magnifying glass held slightly above the eye lens. 

 The appearances observed can be compared with the 

 results of the first set of experiments with a bull's-eye, 

 and they are most important as indicating whether the 

 whole of the N.A. of the objective is being utilised or not. 

 They are as follows : 



(a) When the condenser is focussed the back lens of the 

 objective, or the Ramsden circle, is filled with light if 

 the N.A. of the condenser is large enough. 



(6) When the condenser is racked down from its focussed 

 position shadows appear round the Ramsden circle, and 

 the full N.A. of the objective is not used. 



(c) When the condenser is racked up shadows appear 

 towards the centre of the circle, and the entire aperture 

 is again not used. 



(d) When the iris diaphragm is closed down a black 

 ring appears round the Ramsden circle, and the amount 

 by which the aperture of the objective is restricted can be 

 estimated by comparing the breadth of this ring with the 

 radius of the illuminated part of the circle. 



(e) When the condenser is racked down until the iris 

 is in focus, and the diaphragm then opened, an image 

 of the radiant can be seen in the Ramsden circle. 



Errors in centration of the lamp, mirror, or condenser 

 can be detected by the above examinations of the 

 Ramsden circle. 



6. The Appearance of the Object Field. Having made 

 the adjustments described to centre the condenser, and 



