PHYSICAL ASPECTS OF IMAGE FORMATION 



15 



are larger than blue ones as diffraction gives rise to slight chromatism. 

 This effect is hardly perceived in the central disk. Conversely, it may 

 be detected in the first ring which is white in the central area but 

 slightly iridescent along the edges. Such iridescence is red along the 

 outer edge and blue along the inner. 



Fig. 1.16. Chromatism produced by diffraction. 



Two Airy's disks are shown in Fig. 1.16: they correspond to the 

 radiations C(0-56/i) and F (0-486 /<), respectively. Abscissae show the 

 relative widths of the two central disks, on the basis of identical 

 intensity in the disk centres. This figure only shows the disk en- 

 largement due to diffraction. As mentioned above, obtaining the 

 actual ordinates requires taking into account the spectral charac- 

 teristics of both source and receiver. 



Figure 1.17 shows actual aspects when the source has an equivalent- 

 energy spectrum and the receiver is an average eye whose spectral 

 sensitiveness is given by the Gibson and Tyndall curve (Fig. 1.18). 

 The C and F radiations are close to the ends of this curve and, there- 

 fore, but slightly efficient from the visual standpoint. Comparatively, 

 the diffraction disk, corresponding to peak eye sensitiveness (0-555 /n), 

 would have an intensity equal to 18-7 at the centre (Fig. 1.17). 



These results do not apply to lens-type objectives the chromatism 

 of which is never perfectly corrected. In order to compare the chro- 

 matism of microscope objectives, the chromatism curves, viz. the focal 

 points (perfect images), are shown versus the wave-length. Such curves 

 are not adequate to define image quality as the latter depends not 



