DK. A. E. H. TUTTON: 



L 



rectilinear dark interference bauds on a brilliantly illuminated background in the 

 colour corresponding to the selected wave-length. 



In the instrument now described, one of these two reflecting surfaces concerned in 

 the production of the interference bauds is carried by, and moves absolutely with, one 

 of the two microscopes employed to focus the fiducial marks, or " denning lines," 

 determinative of the length of the standard, the other surface being absolutely fixed. 

 The movement of either of the surfaces with respect to the other causes the inter- 

 ference bands to move, and the extent of the movement of the surface is equal to half 

 the wave-length of the light employed for every interference band that moves past a 

 reference mark carried bv the fixed surface. The movement of the microscope 



Fig. 1. General'view of the comparator. 



parallel to itself and to the length of the standard bar is thus measured by counting 

 the number of bands and the initial and final fractions of a band which are observed 

 to pass the reference spot during the movement, arid multiplying that number by the 

 half wave-length of the light radiation used in the production of the bands. It is 

 only necessary, therefore, in order to compare the lengths of two bars, (l) to place the 

 bar of known length, say, the Imperial Standard Yard, under the two microscopes so 

 that the two defining lines are adjusted in each case between the pair of parallel 

 spider-lines carried by each of the micrometer eye-pieces ; (2) to replace the standard 

 by the copy to be tested, so that the defining line near one end is similarly adjusted 

 under the corresponding microscope, then, if the other defining mark is not also 

 automatically adjusted under the second microscope which carries the interferometer 



