PHYSICAL ASPECTS OF IMAGE FORMATION 9 



Spurious diffraction fringes making observation uncomfortable. Ob- 

 turation also greatly reduces the definition of extended objects of low 

 contrast, this conducina to reduce the diameter of mirror m as much 



Fig. 1.8. (Ij Airy disk; (2) Diflfraction pattern with a reflecting objective. 



as possible. Provided the masked area does not exceed 10 per cent 

 of the whole area of the wave Z-, it can be assumed that the 

 diffraction phenomenon is not substantially altered. 



4. IMAGE OF A LUMINOUS POINT WHEN THE MICROSCOPE IS NOT 

 PROPERLY FOCUSED (MONOCHROMATIC LIGHT) 



The foregoing images (§ 2) require an excellent objective and very 

 precise focusing. Now, microscopists frequently alter focusing during 

 observations in order to ascertain that any particular detail is seen 

 under the best conditions. Therefore, it is desirable to go into the 

 diffraction-pattern changes taking place when focusing is not carried 

 out in the plane of the perfect image but slightly in front of or behind it. 



Let us consider Fig. 1.9 which is a simplified version of Fig. 1.2. 

 As the microscope is no longer faultlessly focused, phenomena occur 

 as if the eyepiece were focused on the plane tz at the short distance .v 

 from 71. The luminous vibrations, diffracted by the various points 

 of the surface wave iT., do not reach the point B' at the time because 

 said point is not the centre of the sphere iT-. Point B' is the centre 

 of sphere 27/ which is removed from sphere 27. by a distance J that 

 increases as the wave edge is neared. The vibrations, originated by M, 



