100 Modern Microscopy 



that are visible. Under these circumstances that central 

 portion of the central disc in no degree contributes in 

 enabling the detail to be seen, but only produces a haze ; 

 by blocking it out the haze is removed, and there is a 

 great improvement in the resulting definition. This pro- 

 duces oblique illumination in all azimuths. 



The Choice of a Condbnsee. — ^That the condenser is an 

 absolute necessity cannot be too strongly impressed. No 

 good results can be obtained without it. 



Condensers,like objectives, not only vary in aperture, but 

 also in power, and the higher the power of the condenser 

 the smaller will be the image of the lamp-flame that it 

 transmits. Consequently, if a condenser of high power is 

 used with a low-power objective, the illuminated portion of 

 the field will be exceedingly small, while if a low-power con- 

 denser is used with a high-power objective, the image of the 

 lamp-flame is so magnified that the whole field is bright, 

 and it is not easy to tell when the condenser is exactly 

 focused. Furthermore, under such circumstances as the 

 latter, it is impossible to get the best effect with the objec- 

 tive. It has usually been recommended that two con- 

 densers, one of high and the other of low power, should be 

 included in a complete equipment, but the new types of con- 

 densers previously referred to cover such a large amount 

 of ground, that the average microsoopist really requires only 

 one. Choose a condenser that gives an aplanatic cone of 

 •90 N. A, and if the major portion of the work is to be with 

 objectives of low and medium magnifications, one of low 

 power should be selected ; if principally with high powers, 

 a corresponding high-power condenser will be necessary. 

 It must be remembered that by removing the uppermost 

 lens by unscrewing its cell, the remaining combinations of 

 a high-power condenser form a very serviceable low-power 

 condenser. 



Oil immersion condensers are of especial utility where 

 objectives of the largest aperture are employed, and these, 

 again, in several instances work well as dry condensers, and 



