22 



PROGRESS IN MICROSCOPY 



isophotal lines shown in the same plane would be co-axial circum- 

 ferences. 



Provided the axial stigmalism requirement holds good at points B 

 and 5,',. the objective meets Abbe's sine condition and is coma-cor- 



FlG. 1.24. Diffraction pattern in presence of coma. Isophotes after Nienlmis. 

 system is very diaphragmed the image is located at B'n. 



the 



rectcd. This applies to microscope objectives. Such aberration is not 

 readily observed in microscopy. With some objectives, it may be 

 detected but may frequently be mistaken for astigmatism. 



Field curvature. Astigmatism 



The image A'^^B'^^ of a small flat object AS, as shown by a microscope 

 objective, is not flat in general: the image is no longer located on the 

 plane rr but on a surface of revolution the concave side of which is 

 on the objective side. The objective exhibits some field curvature which 

 is an aberration inherent in all microscope objectives. From the stand- 

 point of pattern-figure structure il acts as a focusing defect does. 

 Field curvature is not objectionable in anicroscopy in the case of visual 

 observation which is frequently restricted to a smaU detail in the field 

 centre. Conversely, the use of plane-field objectives, the planachomats, 

 is very effective in photomicrography. 



When observing the image B[^ of the luminous point B when the 

 former is close to the edge of the field o^ view, no focusing adjustment 



