12 



PROGRESS IN MICROSCOPY 



decreases intensity in the centre. If the eyepiece is properly focused 

 on a plane lying at a distance from the perfect image equal to B' A'(^, 

 the disk-image appears as shown in Fig. 1.12. Figure 1.13 evinces 

 that diffraction patterns exhibiting dark centres in front of and at 



FiG. 1.12. Diffraction pattern when J = /. 



Fig. 1.13. Light-intensity changes in the centre of the dittVaction pattern when focusing 



is aherecl. 



the back of Airy's disk are again recurrently met with. However, the 

 intensity peak values in the disk centres become very low. These 

 phenomena can be conveniently observed by means of a his^h magni- 



