The Able Diffraction Theory. By J. W. Gordon. 355 



mutually cancel and reinforce one another so as to produce the 

 alternate light and dark spaces, which thus diversify the principal 

 focal plane, a large proportion will be caught by the ocular and enter 

 into new combinations in the plane of its working focus, to build up 

 there the microscopic image of the grating on the stage. In this 

 respect, however, they will not differ from the rays which meet to 

 part again in the central image of the source of light. It is a 

 general rule that all the rays given out from a luminous point which 

 fall upon and pass through a properly corrected lens are reunited in 

 the focussed image of that point produced by the lens. A ray of 

 diffracted light differs for this purpose in no respect from any other 

 ray of light. It is refracted in exactly the same way and to exactly 

 the same point as what has been somewhat obscurely called a "direct" 

 ray along the same axis. In fact it is a direct ray, the direct ray 

 from its source in its own direction, and it is called diffracted, not in 

 virtue of any distinctive property which itself possesses, but in virtue 



Fig. 65. 



of the fact that a neighbouring ray belonging to the same pencil of 

 rays to which it belongs has been suppressed by interference, and that, 

 having strayed into the geometrical shadow of some opaque object, it 

 has escaped the destructive agencies which guard the integrity of 

 shadow margins. It is, of course, true that shadows which thus suffer 

 invasion from straying beams of light retaliate by transgressing the 

 boundary on their own account, and produce straying beams of 

 shadow — if the expression may be allowed — within the illuminated 

 area. But with such diffracted shadows we are not now concerned. 

 It is the diffracted light only which has received attention in the dis- 

 cussion of the Abbe theory, and for present purposes it will be a 

 sufficient description of this diffracted light to speak of it as light 

 which transgresses the limit of a shadow. 



Returning to our diffraction spectra in the tube of the Microscope, 

 we may observe that they lie in what may be quite correctly described 

 as the shadow of the diaphragm of the objective. The accompanying 



diagram will make this clear. 



In fig. 65 the diaphragm is shown 



2 B 2 



