EFFECTIVE APERTURE OF CONDENSER 307 



a greater distance than their marginal rays. If we rack up the 

 condenser so that the marginal rays are focussed on the object, the 

 focus of the rays which pass through the centre will be beyond the 

 object. 



It is well known to those practised in microscopy that, in the 

 case of a narrow cone from a well-stopped-down condenser that is, 

 a condenser used with diaphragms of relatively small diameter the 

 illumination is at its greatest intensity when the object is at the 

 apex of the illuminating cone, and, if the condenser is racked either 

 up or down, the intensity of the illumination is rapidly diminished. 

 But in the case of a condenser with great aperture, if it be racked 

 up, the marginal rays will have their full intensity, while those which 

 pass through the central portion of the condenser will have a 

 diminished intensity. 



The extent to which this wil> take place will be wholly dependent 

 on the amount of under-correction present in the condenser. In 

 some condensers the under-correction is so serious that to obtain a 

 wide or even a moderate cone we so enfeeble the central cone as to 

 reduce it almost to a mere annular illumination, which is not a desir- 

 able quality. 



It will be seen, then, that the aperture of the cone of light trans- 

 mitted by a condenser plays a very important part in giving critical 

 quality to an image with different objectives. We should therefore, 

 to use a condenser accurately, be able to determine the aperture of 

 the cone we are using. 



We may measure the total aperture of a condenser just as we 

 do that of an objective, viz. by means of Abbe's apertoineter. 1 But 

 the effective aperture cannot be measured in that way ; that is to 

 say, the aperture of the largest aplanatic cone (or cone free from 

 spherical aberration) the condenser is capable of giving, cannot be so 

 discovered. 



To do this, place the condenser in the sub-stage and an objective 

 on the nose-piece ; focus both upon an object. Let the edge of the 

 lamp-name be used, and so arrange the focus of both optical com- 

 binations that the edge of the clear image of the lamp-flame falls 

 centrally upon the object. Now move the object just out of the 



FIG. 250. FIG. 251. FIG. 252. FiG.253. 



field, remove the eye-piece and examine the back of the objective, 

 and if the aperture of the aplanatic illuminating cone is greater than 

 that of the objective it will show the back lens to be full of light (fig. 

 250). Therefore, if the aperture of the objective is '5, we know that 

 the aplanatic illuminating cone cannot be less than "5. If now we 



1 Chapter v. 



x2 



