PHYSICAL ASPECTS OF IMAGE FORMATION 



13 



ficatioii substantially diaphragmed-down objective in conjunction with 

 a powerful eyepiece. 



Figure 1.14 shows the results achieved. The isophotal lines, close 

 to A'q, are plotted on the plane MoA'^M (Figs. 1.1, 1.2, 1.3) but, as 

 the phenomena are of revolution about the centre-line AAq of the 

 microscope, results are identical on any plane provided the latter 

 passes through AA'^^. In order to visualize the phenomena occurring 

 in space. Fig. 1.14 should be rotated about the axis AMqA'q. Cutting 

 the resulting volume by a plane at right angles to AA'^, passing 

 through A'q (shown as n in Fig. 1.14), will evince Airy's disk. The 

 straight lines M'A'^^ and MA'f^ are light-rays hmiting the geometrical 

 beam. The dark dots, plotted on the figure, show the positions of 

 zero minimums of Airy's disk (dark rings). Correlation with Fig. 1.5 

 can be established since intersection of isophotal Hnes with AA'q show 

 the intensities obtaining along the microscope centre-line. The dark 

 dots, plotted along AA'^^, show the position of dark-centre diffraction 

 patterns and correspond to points B[, B'^ ... B[' , B.J ..., in Fig. 1.13. 



Fig. 1.14. Isophotes in a meridional plane (After E. H. Linfoot and E. Wolf). 



Figure 1.14 shows at once that, on either side of the plane .t, 

 diflfraction-patterns are identical, unless aberrations of the microscope 

 objective are inadequately corrected. This is a simple and accurate 

 way of evaluating the quahty of an objective. Let us take the above- 



