MEASURING DRY MASSES WITH INTERFERENCE MICROSCOPES 219 



of parallel beam (pin-point source at the condenser focus) which is 

 normal to the specimen. To obtain sharper images, opening the con- 

 denser diaphragm is always beneficial. Under such conditions, the 

 specimen is illuminated by numerous parallel-ray beams, slanting 

 at various angles. In Fig. 8.10 the diaphragm D of the condenser C 

 is reduced to the very low aperture So, located in the focus of C. 



So 



Fig. 8.10. The specimen is illuminated by a parallel beam (low condenser aperture). 



The specimen is illuminated by a parallel beam. In the case shown 

 in Fig. 8.11, when the diaphragm D is opened to a greater extent, 

 any point S of the diaphragm aperture sends forth, through the 

 specimen, a parallel-ray beam whose slant ao varies as does the po- 

 sition of point S. Phenomena are identical in reflected light (Fig. 8.12). 

 If the diaphragm of the vertical illuminator (not shown in Fig. 8.12) 



Fig. 8.11. The specimen is illuminated by a parallel beam whose slant a varies as does 



the position (diaphragm D opened). 



is almost closed, phenomena occur as if there were a pin-point radiant 

 Sq at the focus of the objective Oj. The specimen P is illuminated 

 by a parallel-ray beam. If the diaphragm of the vertical illuminator is 

 opened still further the imaged source does not consist of a point at So 

 but of an extended area. A random point S of this image sends a beam, 

 of slant ao on to the reflective object P. Assuming a be the maximal 

 value of tto in Fig. 8.11 and 8.12, it is the slant of the beam when the 



