PHYSICAL ASPECTS OF IMAGE FORMATION 



47 



the light transmitted to S'q passes into the objective. The image plane P 

 illuminated solely by the source S'^ is evenly illuminated. The field 

 of view perceived by the eye from eyepiece 0.>, is uniform and free 

 from the imaged grating. Assuming that the N.A. of objective O^ is 

 adequate to let through image S^ and spectra S[ and S!,, the plane P' 



Fig. 1.59. Image of a periodic object when the N.A. of objective O, lets through ^o and 



spectra S^ and So. 



is then illuminated by three coherent sources: S'^^, S[, S!,. Curve 1 

 in Fig. 1.59 shows the image structure. The pecked curve 2 shows 

 the geometrical image of the grating. Instead of a sharp definition 

 of the bright lines of the grating, the image exhibits shaded-off edges; 

 the image period remains the same as that of the grating. Let us 



Fig. 1.60. linage of a periodic object when the N.A. of the objective O, lets through 



Sq and two spectra on either side of S^. 



increase the objective's N.A. to let in iwo spectra on either side of .S^. 

 The image then exhibits additional diffraction fringes (Fig, 1.60). 

 As more spectra are let in, the image period does not alter and remains 

 that of the object grating but the outline image gradually tends to 



Fig. 1.61. As more spectra penetrate into the objective, the image looks more and more 



like the object. 



a perfect image (Fig. 1.61). The geometrical image is the limit-image 

 achievable were all the spectra involved, that is, with an objective of 

 infinite aperture. If but few spectra are let in, the luminous con- 

 tinuities are not shown in the image and substituted for shaded-ofif 

 outlines that do improve sharpness of the image. As more spectra 



