ILLUMINATION OF OBJECTS; ABBE CONDENSER 



23 



employ oblique illumination, and finally place diaphragms 

 between object and mirror, noting well any changes in the appear- 

 ances of the images seen. 



DEVICES FOR ILLUMINATION BY TRANSMITTED LIGHT. 



Condensers. — In order that sufficient light may enter a high- 

 power objective to produce an image of such a degree of bright- 

 ness as to be easily studied, it is essential that some device or 

 apparatus shall collect, concentrate and send through the object 

 light rays at an angle which will fill the aperture of the objective. 



The usual construction of this device is shown in diagram in 

 Fig. 4 and is known as the Abbe condenser. Condensers of this 

 construction with two lenses 

 have usually a numerical aper- 

 ture, when employed to their 

 full extent, of 1.20 and may be 

 used with all ordinary dry ob- 

 jectives and with oil immersion 

 objectives. They are designed 

 to be used with the plane mir- 

 ror. In the case of objectives 

 of more than 1.20 N.A., a three 

 or more lens combination con- 

 denser giving 1.40 N.A. should 

 be chosen. Condensers used 

 to their full aperture usually so 

 flood the field with light, in the 

 case of dry objectives, as to 

 necessitate lowering them or 

 closing their iris diaphragms 

 or both until only just sufficient 

 light rays are intercepted by 

 the objective to fill its back 



Fig. 4. 



Diagram of Abbe Condenser; 

 Axial Light. 



lens and thus render the fine details of the illuminated object 



most distinct. 



In the diagram, Fig. 4, the passage of the light rays is roughly 

 indicated for a position of the Abbe condenser when used with 



