24 PROGRESS IN MICROSCOPY 



why the geometrical aspect of these two aberrations are never observed 

 in microscopy. Their presence is only detected by shght alterations 

 of the diffraction pattern. On the other hand, shght astigmatism 

 affects substantially the diffraction pattern which vanishes and the focal 

 hnes of geometrical optics are evinced. Astigmatism is a common aber- 

 ration in microscopy and is readily observed with high numerical apert- 

 ure and low-magnifying objectives. It is advisable to ascertain that the 

 astigmatism observed is not originated by the eyepiece which is 



Fig. 1.27. Isophotes when spherical aberration, coma and astigmatism are present (after 



A. Marechal). 



a common occurrence. While coma and astigmatism are tolerable on 

 the edges of the field of view such is not the case for the centre because 

 such aberrations would be due to the lens objective's being out of line. 

 Such an objective is to be definitely rejected. When coma and astigma- 

 tism are present concurrently, the isophotal curves in the diffraction 

 pattern may take on rather involved shapes. Figure 1.27 is a typical 

 example of such a combination of aberrations. 



7. OBJECTS IN COHERENT, INCOHERENT AND PARTIALLY COHERENT 



ILLUMINATION 



Optical objects termed "incoherent" are objects the various elements 

 of which emit fully incoherent vibrations, i.e. independent from one 

 another. Such objects, luminous per se, such as the sun, the various 

 terrestrial sources, landscapes, are incoherent objects. The various 

 monochromatic radiations forming the spectrum from a source are 

 incoherent from one another. How emission of luminous vibrations 

 takes place is still an uncompletely solved problem but it is known 



