CONDENSERS 103 



chromatic errors are corrected with an accuracy approach- 

 ing that of objectives. Parallel rays passing through 

 different zones of an imperfectly corrected condenser 

 are not focussed to the same point, but there is a larger 

 or smaller area around its centre from which the rays 

 focus very closely to one point. The cone made by these 

 rays is called the " aplanatic cone " of the condenser, 

 and it increases in proportion as the spherical errors of the 

 condenser are eliminated, until rays from all zones focus 

 to one point, when its aplanatic cone equals its numerical 

 aperture. At the apex of this cone all the available 

 light is brought to a focus, whereas at no other point 

 within or without the cone is this the case. As there is 

 no such definite point with an uncorrected condenser, 

 it cannot utilise the full power of the radiant so efficiently, 

 and the exposure that even a good one necessitates 

 may be as much as one-third greater than is needed when 

 a corrected condenser of the same aperture is used. 

 Besides giving a cone that is far from aplanatic, un- 

 corrected condensers are not achromatic, and they 

 usually produce colour-fringes round the disc of light 

 they project, and the outer zones of any area on which 

 the light falls are therefore very imperfectly illuminated. 

 Power of a Condenser. Condensers are made of varying 

 focal length according to the use for which they are 

 intended, and the size of the image of the radiant that 

 they form increases with the focal length. The focal 

 length of the condenser used must be sufficient to ensure 

 an even illumination of the entire area of the aperture 

 of the substage condenser, or field of the objective, as the 

 case may be. On the other hand, the N.A. and intensity 

 of illumination increase as the focal length decreases, 

 and these must be as high as possible, particularly in the 

 case of the substage condenser, as the N.A. of the objective 

 cannot be fully utilised unless that of the illuminating 

 cone is adequate. The power of the condenser must 



