PHYSICAL ASPECTS OF IMAGE FORMATION 



17 



Results are shown by the curves in Fig. 1.20. Abscissae evince, 

 in the focusing plane, the distance (in microns) to the geometrical 

 centre of the image while ordinates show the luminous intensities, 

 taking as intensity unit that obtaining in the centre of the diffraction 

 pattern produced by the apochromatic objective. In the three ob- 

 jectives, then, //sin// = 095 and g = 60. 



Fig. 1.19. Spectral distribution of energy of the photometrists standard A. 



The maximum spectral distances from C to F (longitudinal chro- 

 matism), selected for calculations, are: 



apochromatic objective x = 1 mm 

 fluorite objective x = 4 mm 



achromatic objective x = 8 mm. 



Curve (a) shows the image structure afforded by the apochromatic 

 objective. Ordinates, in this and the following curve, were multiplied 

 by 40 from the abscissa 20 onwards. There are no zero minimums 

 any longer. Were the image, shown in this figure, produced by an 

 objective completely devoid of chromatism, e.g. a reflecting objective, 

 the differences would be rninute. Central peak intensity would be 

 1015, while that of the first luminous ring would drop by 0001. 

 Hence, the difference between an apochromatic and a perfect objective 

 is very sHght. In both cases, vanishing of the dark rings is due to 

 a change in diameter of the diffraction disks taking place in accordance 

 with the wave-length. Therefore, effects of the residual chromatism 

 of the apochromatic objective are less prominent than those of 

 diffraction-originated chromatism. 



