Theories of Microscopical Vision. By A. E. Conrad i/. C17 



in the conjugate point of a Microscope focussed on the grating. 

 But when the slits are of a sensible width compared to the dark 

 interval, there will be differences of phase between the light pro- 

 ceeding from different portions of any one slit ; there will, there- 

 fore, be interference between these portions, and the resulting 

 combined phase remains to be determined. 



Let A and B (fig. 95) represent the centres of two adjoining 

 slits of a grating, then we know already that the light proceeding 

 from these points joins up with a difference of phase of one or 

 several wave-lengths to form the plane diffracted wave C D. 



If we now consider two points equidistant from the centre A, 

 it is evident that the light emanating from these will be out of 

 phase with that starting from A, and that the difference of phase 

 between the light from either of those two points and A will be in 

 the same proportion to the difference of phase of one or several 



whole wave-lengths between C and D as E A = F A is to A B. 

 The resulting phase will be determined according to the principle 

 of Huyghens, by adding together the disturbances of all the 

 luminous elements of surface contributing to the wave C D, and 

 discussing the resulting integral wave. I will deduce the result 

 in an elementary but correct manner by a rough mechanical inte- 

 gration, instead of the mathematical one, which might repel many 

 of my readers. 



The disturbance caused, say at C, by the light coming from A 

 may be represented by the simple formula — 



(1) x x = c . sin a, 



where a is an angle uniformly increasing by 360° for every com- 

 plete vibration, and c, the amplitude of the wave, is a constant 

 •depending on the intensity of the light, the formula simply imply- 

 ing that the vibrations of li^ht follow a sine law. The light from E 



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