PHYSICAL ASPECTS OF IMAGE FORMATION 



11 



showing changes in intensity when focusing is altered in the centre 

 of the diffraction disk. Results are shown in Fig. 1.13. Focusing 

 changes are made by fine focusing movements but Fig. 1.13 can be 



Fig. 1.10. (1) Airy disk; (2) Diffraction pattern when focusing is but slightly defective. 



Fig. 1.11. Diffraction pattern when defective focusing has a value smaller than ).. 



interpreted more readily assuming that it was the eyepiece that moved. 

 If the eyepiece is focused at A[^ peak intensity /q is in the centre of the 

 diffraction pattern. The curve (Fig. 1.13) shows that defective focusing 



