PHYSICAL ASPECTS OF IMAGE FORMATION 



21 



the curves in Fig. 1,22 show that the diffraction pattern in the plane 

 -Ti diverges from the diffraction pattern in the symmetrical plane .t.^. 

 As mentioned in § 4, the differences in appearance observed on each 

 side of the good image enable one to detect the objective's spherical 

 aberration. When focusing on the side of the plane n[, diffraction 

 disks of varying complexity exhibit a fairly large number of diffraction 

 rings. When focusing on the other side of n, a nearly uniform halo 

 is visible, which grows larger as the focusing plane recedes from n. 

 This method does not allow one to measure aberration quantitatively 

 but merely to test it. However, it can be said that, if a small divergence 

 in appearance is evinced to a trained eye, the objective's spherical 

 aberration equates approximately the path difference /1/4. 



Coma 



When the pin-point object and its image A'^^ are along the axis 

 (Fig. 1.1) and provided that the objective be corrected for both these 

 points, the image A'q is Airy's disk. The objective is stigmatic at 

 points A and A'^. 



Let us now consider a luminous, pin-point object B, away from 

 the axis at a short distance from A (Fig. 1.23). If the stigmatism shown 



Fig. 1.23. Object B and image B^ out of the axis. Coma. 



at points A and A'^^ does not hold good for both B and its image 5^, 

 the wave I\' , related to points B and 5^, is not spherical. The ^^ image 

 is distorted and not of revolution; diffraction rings are no longer co- 

 axial and the image has only one plane of symmetry. The image B'^ is 

 shaped as a comet-tail and exhibits coma. Figure 1.24 shows the 

 aspect of the diffraction pattern in presence of coma in a plane per- 

 pendicular to the microscope axis. The plotted lines show isophotal 

 lines. In the case of the ideal diffraction pattern (Airy's disk) the 



