A WAVE-LENfiTH COMPARATOR FOR STANDARDS OF LENGTH. 7 



the telescope. These wedge-discs are mounted close together, apparently, but not 

 quite, parallel to each other and to the black-glass disc carried by the microscope. 

 The surface of the colourless disc, g 2 , nearest to the black-glass surface, and the 

 latter surface, y a , are the two surfaces whose reflections are made to interfere. The 

 reflection from the back surface of this colourless disc is got rid of by its 35 minutes 

 of inclination. The back surface of the black-glass disc is ground. 



The other colourless-glass disc, </,, acts as a countervailing wedge to neutralise the 

 slight dispersion introduced by giving the second surface of the first colourless disc, 

 g 2 , 35 minutes of inclination. This it does absolutely, being cut from the same large 

 35-minute slab. It is slightly tilted in a direction 90-degrees to the direction of the 

 wedges, which are placed with the thinner end of one opposite the thicker end of the 

 other. Both discs are carried in a fitting, G, which provides for further separate 

 adjustment by three screws in each case. 



(5) A \l-and-plane bed, t, similar to that of (2), rests 7f- inches lower than, and 

 parallel with, the latter in a step cut for 7|- inches backwards in the stone to replace 

 its top front edge. This bed rests like (2) in a basal plinth, u, bolted down to the 

 stone. 



(6) An adjustable table, v, for the support of the standard bars, B] and B 2 . This is 

 arranged to slide over the bed referred to in (5), and is provided with both quick and 

 fine adjustments for its horizontal position on the bed, with a transverse motion 

 perpendicular to its length, a fine adjustment for azimuth, and another for levelling. 

 There are three interchangeable table tops. One is merely a truly plane experimental 

 top ; a second is fitted with friction rollers in the officially approved manner for the 

 support of line measures ; a third provides for the similar support of both line and 

 end bars. 



(7) A fine-movement control wheel, w. This is supported on a special bearing- 

 pedestal, x, near the observer's left hand, and to the left of the telescope. It is used 

 for bringing about the fine movement of the microscope, M 2 , which carries the black- 

 glass surface, </ 3 , relevant to the interference. The wheel has a diameter of 6 inches 

 and a circumference of approximately 1 9 inches. It is rigidly connected, by means of 

 either a solid or a flexible steel shaft, y, with the axis of the endless screw ,s- 

 referred to in (3). Each complete rotation of the wheel corresponds to the traverse 

 of the microscope and its black-glass disc for 0'005 mm., which is equivalent to the 

 passage of 1 5 interference bands of red C hydrogen, or red cadmium light. Hence 

 more than an inch of movement of the circumference of this wheel occurs for the 

 passage of each interference band, thus affording very considerable delicacy of control 

 over the passage of the bands. 



Further details essential as elucidating the working of the instrument, or because 

 they involve original mechanical devices, will be given, after a brief account of the 

 foundations and of the thermostat, under the headings of the seven parts, whose 

 main functions have now been indicated. 



