Dr. G. J. Stoney on Microscopic Vision. 



505 



the inclination of the beam to the optic axis. All points of 

 these lines are in the phase 6 at the time t. 



Proceed in the same way with another beam whose inclina- 

 tion to the opic axis is a', and which lies in a plane of incidence 

 inclined at an angle <f> to the plane of incidence of the first 

 beam. It produces on the image plane a system of parallel 

 lines in the phase 6 at the time t ; which lines stand at 

 intervals S / =\'/sm a' \ and are inclined at the angle </> to the 

 first set. 



Let fig. 1 represent the image plane with the two systems 

 of lines drawn upon it. They form parallelograms ; and if 

 we draw the dotted lines of the figure, which are diagonals 



Fig- 1 



to these parallelograms, we find that the one beam is always 

 in the same phase as the other at every point of these dotted 

 lines — in other words, these dotted lines are the middle lines of 

 the equidistant luminous bands which constitute the riding 

 produced by the two beams on the image plane. 



Let a be the spacing of this ruling, i. <?. the interval 



