54 



PROGRESS IN MICROSCOPY 



limit is much the same as in incoherent illumination (expression 1.10). 

 The curve shows that resolving power is best when ihe condenser 

 aperture is 1-5 times higher than the objective's. However, in actual 

 practice, owing to spurious light, this theoretical result is not ap- 



F IG. 1.70. Aspect of an object consisting of luminous points on black ground: coherent 



illumination. 



plicablc. Experience shows that when (he ratio /// 6 or 7, it can 

 hardly be exceeded without substantially increasing stray light and, 

 concurrently, decreased image contrast. 



The phenomenon is the same for a periodic object: resolving power 

 improves as the condenser aperture is increased. In Fig. 1.72, illumi- 

 nation is coherent, the condenser lens is almost fully diaphragmed 

 and phenomena occur very much as if the light were originated from 



