356 



Transactions of the Society. 



without any lens, and passing a cone — sensibly a cylinder — of light, 

 having its apex at the virtual source, its base at an infinite distance 

 behind the opening of the diaphragm. In fig. 66, the same diaphragm 

 is shown fitted with a lens which condenses the emergent beam into a 

 sensible cone, having its apex at the principal focal point, so that the 

 opening of the diaphragm becomes the base of one sheet of the 

 emergent cone, the other sheet having its base a3 before at an infinite 

 distance behind. The effect upon the shadow, as shown in the 

 diagrams, is to vary its contour, but physically the shadow itself 

 remains unchanged. In both cases it is produced by the destructive 

 interference of one part of the transmitted wave-front upon the other 

 part, and in both cases the same — i.e. essentially the same — boundary 

 phenomena are manifested at and near the confines of the beam. 

 Geometrical conditions which cause an intensification of the central 

 light at the principal focal point give rise to similar intensification of 



Fig. 66. 



the diffracted light at the angular focal points marked S in the figure, 

 and thus render visible and even conspicuous phenomena which the 

 diffused light surrounding the shadow cone in fig. 65 is too weak to 

 show. 



In order to appreciate the use made of these phenomena in the 

 Abbe theory, it will be convenient to add a diagram, fig. 67, in which 

 the shadow cones originating on the stage are shown in comparison 

 with the shadow cones originating at the source of light. By these 

 means it is possible to exhibit to the eye the course of the diffracted 

 light, and to indicate what part of it, after having contributed to the 

 spectrum images of the source of light, goes on to contribute again to 

 the building up of the image of the olg'ect on the stage in the visible 

 focus of the Microscope. Thus the diffracted rays contribute to 

 the formation of a microscopic image upon the ordinary theory of 

 image formation. In this way, for example, they will, by common 

 consent, when present contribute to the formation of the broad out- 

 lines of the microscopic picture. 



The Abbe theory attributes to these rays of diffracted light a 



