130 



PROGRESS IN MICROSCOPY 



illumination changes tending to obfuscate interpretation if the object 

 is concurrently a phase object of varying thickness (reflecting evenly 

 the light) and an ampHtude object (featuring illumination or colour 

 changes). 



Figure 4.3 shows that the Kohler illumination is apphcable to 

 both methods. The lens Li images the source 5 on the lens Ci. The 

 lens Co then images Q at F\ which is the focus of the objective O^, 



Fig. 4.3. Kohler illumination in reflected light. 



after being reflected in G. The object P is thus evenly illuminated. 

 The lenses Q, Co and the objective O^ image the field diaphragm D^, 

 placed against Li on the object P. The whole system SL^CiCoG 

 forms an element integral with the microscope and is a vertical illumi- 

 nator. In other layouts, the vertical illuminator consists of CiCo 

 and G only. The source 5 and the lens Lj, belong to an independent 

 system. 



The diaphragm D.> is placed against C^ (diaphragm aperture) in 

 order to vary the illumination angle. The system that exhibits the 

 most stray light is that shown in Fig. 4.1. A ray such as SIJ (Fig. 4.4) 

 is reflected at / in one of the objective's lenses and reverts to the 

 eyepiece. The stray light is originated owing to glass reflection from 

 the surfaces of the objective's lenses which may attain 7 in number 

 in powerful achromats. Stray light is, therefore, substantial: much 

 more so than in transmitted-Iight microscopes as, in the latter, the 



