1913] BRUSH— SOME DIFFRACTION PHENOMENA. 281 



by slowly turning the tangent screw, the fringes move closer together 

 and lose their uniform spacing and most of their color, while the 

 sharpest and blackest bands move further out in the pattern. 



The width of the mirror, in the line of sight, may be reduced to 

 2 mm. without affecting the fringes in any respect ; but with con- 

 tinued further reduction the fringes progressively lose their color, 

 increase in number, and assume the characteristic spacing of diffrac- 

 tion fringes strongly reinforced by superposition of patterns, when 

 the width is only a fraction of a millimeter. 



These phenomena are beautifully shown by means of the device 

 illustrated in diagram P. The plane glass mirror is here shown 

 both in plan and elevation and enlarged to the scale of the razor 

 blades A' and R. It is in the form of a thin wedge about 12 mm. 

 long and 3 mm. wide at the base, giving a triangular face. The line 

 of sight is indicated by the dotted line. 



Having adjusted the face of the mirror so as to produce the 

 Lloyd fringes, and with the near edge of the mirror in the focal 

 plane so as to prevent any edge effect, the mirror is very slowly 

 moved on the microscope stage across the line of sight toward the 

 point, without change of angle with the incident light. During this 

 movement all the last described effects are developed. I may add 

 that smoking the face of mirror H or P does not materially affect 

 the brilliancy of the fringes. 



In view of the facts cited it seems clear that the so-called " single- 

 mirror interference fringes " of Lloyd are superposed diffraction 

 fringes, and are not due to reflection. But to remove all doubt the 

 device shown in diagram R was constructed. 



This consists of 24 paper-thin razor blades clamped together and 

 forming a bundle about 4 mm. thick. It is essential that all the 

 edges be accurately brought to the same plane. But inasmuch as 

 the edges of the blades are not perfectly straight, this condition can 

 be realized only in two lines across the edge of the bundle. To effect 

 this adjustment, the edges of the blades, very loosely clamped to- 

 gether, were allowed to rest by gravity against two parallel straight 

 glass rods about half the length of the blades apart, and then cau- 

 tiously clamped tight. Great care was taken to avoid injury to the 



