CLASSICAL THEORY OF LIGHT 759 



straight fringes due to that of D and M. Among these latter the 

 central fringe marking the line of intersection of D with the virtual 

 image of M could hardly be identified; but when the white light is 

 turned on, it stands forth unique. It is made to coincide with some 



JN' 



MM 



Fig. 5. 



fiducial mark in the field of view, and the measurement commences. 

 The red light being restored, the observer transfers his attention to 

 the circular fringes formed by N and D, and counts them as they pass 

 while he moves D along towards the virtual image of M'. Making 

 occasional tests with the white light, he eventually notes the advent 

 of the blaze of colours and the central black fringe which indicate the 

 intersection of this image with D. When the black stripe coincides 

 with the fiducial mark, the reflector D has moved through the length 

 of the standard MM', and this length has been measured by the count 

 of the circular fringes which meanwhile were passing by in the other 

 part of the field of view. The number of waves is, of course, not as a 

 rule an integer; the fractional excess may be estimated quite accurately 

 in various ways. 



In the actual work, this shortest standard was about 0.4 mm. long, 

 and comprised somewhat over 606 waves of red cadmium light. The 

 counting of the 1212 fringes corresponding to its length was the only 

 counting required; the rest of the process consisted of nine stages, 

 the first of which was the type for all the others except the last. 

 This second step was the comparison of the shortest standard with a 



