PHASE-CONTRAST MICROSCOPY 



81 



Now, let us bring the system C closer to the specimen (Fig. 2.16(b)). 

 Owing to the distance from C to L, the incident beam is reflected on 

 to the convex mirror at a'a' in an area more removed from the axis. 

 Instead of travelling back to bb, the light is then reflected direct by 



C d 



As-:--;" 



(b) 



Fig. 2.16. Heine phase-contrast microscope (Leitz). 



the annular ring dd. The emerging light-beams traverse the specimen 

 at very wide angles and do not penetrate into the objective. The light- 

 beams diffracted by the specimen details are the only ones to be 

 collected by the microscope's objective and a standard dark ground 

 is thus obtained. In this way, shifting the system C brings about 

 a continuous change-over from the various following positions: light 

 ground, phase contrast, oblique-illumination, light ground, dark ground. 

 In Messrs. Officine Galileo's phase-contrast microscope, a pancratic 

 system forms an image of variable dimensions from a iixed annular 

 diaphragm. In this way the imaged ring dimensions are adjustable 

 and made to match the phase-plates of the objectives involved. 



