8 PROGRESS IN MICROSCOPY 



enlarged and that niinimums are no longer zero. Enlargement is in 

 the region of 8 per cent for an aperture sinw = 085. Alterations 

 of the diffraction pattern are therefore small and may be neglected 

 in a first approximation. 



3. IMAGE OF A LUMINOUS POINT ORIGINATED BY A REFLECTING 



OBJECTIVE 



It may occur occasionally that a portion of the i7. wave, originated 

 by the objective, is masked by an opaque screen as, e.g. in a reflecting 

 objective. Many types of reflecting objective are used in microscopy, 

 but from the standpoint now under discussion, they may all be likened 

 to the diagram in Fig. 1.7. 



Fig. 1.7. Diagram of the ray paths through a reflecting objective. 



In this type of objective, the hght, originated by the object A. is 

 reflected first by the concave mirror M then by the convex mirror n? 

 and ends at the image A'^^ being examined through the eyepiece Oo. 

 The small mirror m screens the beam which is reflected on M. 



The surface waves -T,, and H^ are ring-shaped and the diflYaction 

 pattern is altered. Curve (1) in Fig. 1.8 shows the distribution of 

 intensity in the conventional diff"raction disk brought about by the 

 whole wave (Airy's disk). Curve (2) shows the structure of the 

 diftYaction disk when the wave is masked in the central area by a 

 circular opaque screen. If the small mirror is kept in position by 

 three thin strips 120' apart, another perturbation of the diffraction 

 phenomenon is developed: light is accumulated in three 120 directions. 



Curve (2) in Fig. 1.8 shows that screening the beam reduces the 

 central disk, which would be beneficial were it not that the luminous 

 rings are intensified. When extended high-contrast objects are dealt 

 with, there is nevertheless a eain, but the intensified rings originate 



