670 Profs. Morley and Miller : Experiments to 



He develops expressions for angles of reflexion, for wave- 

 length after reflexion, and for the conditions which determine 

 the network of parallelograms formed by the two systems of 

 wave-fronts. The diagonals of these parallelograms are 

 alternately lines of maximum and minimum disturbance in 

 the aether, so that they define the interference phenomena. 

 These expressions are not only rigorous, but also general, 

 applying to any adjustment whatever of the optical parts of 

 the apparatus, and form a welcome contribution to the 

 thorough understanding of the theory of the Michelson and 

 Morley experiment. 



In one passage he says that a term added by him " may 

 entirely modify the nature of the changes produced as the 

 direction of the drift alters " ; and some appear to think that 

 the inference from the earlier experiment is involved in 

 doubt by this discussion. It is therefore well to examine 

 again the theory. 



Let D, PL IX. fig. 1, be a plane-parallel glass plate, silvered 

 so thinly that equal quantities of light are transmitted and 

 reflected. S being a source of light, part of the light passes 

 through D, moves on to the plane mirror II, where it is 

 reflected back to D ; here, part is transmitted and lost, part 

 is reflected to the observer at T. The other part of the 

 entering light is reflected at the first surface of D, reflected 

 again by the mirror I, is in part reflected by D, and lost, in 

 part transmitted through D, and proceeds towards T. If 

 distances and angles are suitable, the reunited rays between 

 D and T will produce interference phenomena. If distances 

 are equal, we may obtain interference phenomena in white 

 light. In one of the usual adjustments of distances and 

 angles, parallel fringes are seen when the eye or the telescope 

 is made to give distinct vision of one of the mirrors I or II. 

 The fringes apparently coincide with these surfaces. A 

 central fringe is black ; on either side are coloured fringes, 

 less and less distinct till they fade away into uniform 

 illumination. If the path of either ray is shortened, the 

 fringes move rapidly to one side. If we engrave a scale on 

 I or II, we can, after any alteration of one of the paths, 

 restore with great accuracy and ease the former relations by 

 bringing the central dark fringe to its original place on this 

 scale. If the motion of the earth through the aether were the 

 cause of this change of path, we could measure the amount 

 of change by measuring the displacement. 



Suppose, fig. 2, that the apparatus moves in the direction 

 of the arrow through the stationary aether. While the ray 

 of light moves from D to I and returns to D, the mirror t> 



