ELEMENTARY CHEMICAL MICROSCOPY 



the character of the information thus gained is necessarily 

 closely associated with the resolving power, penetration and, 

 to a certain extent, the size of field of the optical combination 

 above the stage. 



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, concen- 

 trate 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. 9 and is known as the 

 Abbe condenser. Condensers 

 of this construction with two 

 lenses have usually a numerical 

 aperture, when employed to 

 their full extent, of 1.20 and 

 may be used with all ordinary 

 dry objectives and with oil im- 

 mersion objectives. They are 

 designed to be used with the 

 plane mirror. In the case of 

 objectives of more than 1.20 



N.A., a three or more lens com- 

 Fxc. 9 . Diagra^of Abbe Condenser; ^.^ ^^ ^ ^ 



N.A. should be chosen. Con- 

 densers used to their full aperture usually so flood the field 

 with light, in the case of dry objectives, as to necessitate lower- 

 ing them or closing their iris diaphragms or both until only 

 just sufficient light rays are intercepted by the objective to fill 

 its back lens and thus render the fine details of the illuminated 

 object most distinct. 



