A WAVE-LENGTH COMPARATOR FOR STANDARDS OF LENGTH. 23 



hand with the milled head, shows absolutely no backlash or " dead space" on reversal, 

 and the bands follow its rotation perfectly. The flexible shaft shows a little " dead 

 space " due to torsion on reversal, but when once the bands begin to move they follow 

 the rotation of the wheel perfectly and they stop immediately the wheel stops. 

 Neither of the solid shafts show any hesitation on reversal of the wheel's rotation, 

 and the bands stop absolutely when the wheel stops. 



The Method of Production and Adjustment of the Interference Bands. 

 (Fig. 7 will be very useful in following this description.) 



The mode of once for all adjusting the interference apparatus is briefly as 

 follows : 



The black-glass disc y s , carried by the right-hand microscope on its right side, 

 which forms one of the two all-important reflecting surfaces employed for the 

 generation of the bands, has once for all been adjusted to the truly vertical plane, 

 parallel to the axis of the microscope, and perpendicular to the direction of the 

 V-and-plane beds and standard bars. The large total-reflection prism, already 

 adjusted with truly vertical faces on its carrier table, is placed in position on the 

 circle of the dispersing apparatus, dropping automatically by its three pegs into the 

 corresponding holes, and is locked there. The white-light goniometer lamp a 

 Nernst lamp in an adjustable cylindrical copper tube, with a plate of ground glass to 

 diffuse the light closing its aperture ---is placed with the ground glass in front of the 

 collimator instead of the Geissler-tube fitting ; and the image of the little rectangular 

 stop in front of the small total-reflection prism of the autocollimating telescope, as 

 reflected from the black-glass surface, is adjusted in the field of view of the telescope. 

 The common eye-piece only is employed, not the micrometer eye-piece, and the iris 

 diaphragm is left wide open. The visible field is semicircular, the right half of the 

 complete circular aperture being closed by the opaque mount of the little total- 

 reflecting prism, whose left edge is the straight vertical line forming the base of the 

 semicircle. 



The two colourless-glass 35-minute wedge-discs <j l and g 2 are then placed in their 

 spring mounts, the fitting carrying which is already in position. In placing each 

 wedge-disc in position, it is only necessary to hold up the spring, which bears only on 

 the normally ground cylindrical edge-surface of the disc, by a little handle projecting 

 through the mount, and to push the wedge-disc in until it is stopped by the truly 

 planed flange, which occurs when about two-thirds of the thickness of the disc has 

 entered the mount ; and then to release the spring, which maintains the disc firmly 

 pressed in position without chance of movement, and without any bending strain on 

 the all-important surfaces of the disc, any slight strain being only parallel to those 

 surfaces. The disc g a should first be thus placed in position, in the left mount, with 



