Evening Glow and analogous Phenomena. 279 



€ d . . . , Aa', the rays proceeding from the parts of the wave 

 AW and d W produce no effect, and those incident upon the 

 dark screen between A and B are quite kept off. Hence only 

 the part B d of the wave can come into operation. But if m is 

 the centre of c d, tm&dn be made equal to B m, the pencils B m 

 and dn mutually extinguish each other, because they differ in 

 their paths by half a wave-length ; hence there merely remains 

 the pencil m n, which is only distinguished from the pencil c B, 

 remaining in the small aperture, by being displaced about JX. 

 Hence the pencil m n will have the same amplitude but with 

 opposite sign, and exactly the same intensity as the pencil cB. 

 It is only when the ray A S is at right angles to the plane of the 

 wave (for the direct rays, that is), that the construction in ques- 

 tion does not hold. But in this case all direct rays striking the 

 object-glass directly will be concentrated in its focus. 



Nothing in this conclusion is modified if in the figure we take 

 cB>±cd or A«>AB ; and it remains equally applicable if we 

 have to do, not with one aperture and with one screen, but with 

 any given group of apertures or screens. Hence we propound the 

 following principle : — 



The phenomenon of diffraction produced by one dark screen, or 

 by a group of dark screens, is quite identical w it h that arising from 

 an aperture of the same shape, or a group of apertures, with the 

 single exception of that point in which the direct rays unite ; fcr 

 all light is here collected which is not kept off by the small 

 screen. 



The action of a perforated screen is distinguished from that 

 of a group of screens, which, so to speak, is the negative of the 

 former, by the fact that in the first case both the direct and the 

 diffracted light depend on the parts of the screen transmitting 

 light, while in the second case the direct light depends on the 

 bright, the diffracted on the dark parts. In the former case the 

 diffracted light, as regards its intensity, conforms to the direct ; 

 in the latter case there is no such conformity. If the apertures 

 of a dark screen be increased until, for instance, their total sur- 

 face is equal to the contents of the part which remains dark, both 

 0^2 direct and the diffracted light increase in intensity; but if, 

 in like manner, the small screens of a group of small screens be in- 

 creased, the diffracted light is thereby increased, while the direct is 

 weakened. 



4. Hence, if a pencil of parallel rays (a plane wave) from an 

 infinitely distant white point of light impinge on a (plane) group 

 of very small dark screens, an eye behind the group will converge 

 the direct rays to a white image of the luminous point, which ap- 

 appears surrounded by diffracted and, perhaps only imperceptibly, 

 reddish- coloured light. The action of the group of screens is 



