A WAVE-LENGTH COMPARATOR FOR STANDARDS OF LENGTH. 



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total-reflection prism for use in adjusting the apparatus. This and the pair of refraction 

 prisms are seen on their carrier plates resting in the tray at the base of the pedestal 

 in fig. 1. All three carriers may be fixed in position on the circle plate by the simple 

 arrangement of a locking plate fitting into grooves in the three pins, and manipulated 

 by a little projecting handle. 



The black-glass disc, g a , whose outer surface, 1 inch in diameter, is polished an 

 absolutely true plane, which reflects one of the rays concerned in producing the 

 interference bands, is mounted on a bevelled slider, a in fig. 7 (which shows the 

 interference discs more clearly and their mounts), on the side of the microscope tube 

 just above the objective, in a vertical correspondingly dovetailed guiding bed, b, so as 

 to be capable of longitudinal adjustment and eventual rigid fixation at the convenient 

 height. The disc may also be adjusted by means of three screws, c, precisely to the 



Fig. 7. The interference discs. 



vertical plane, perpendicular to the horizontal axis of translation of the microscope on 

 its sliding bed. This is achieved by making the circular mount for the black-glass 

 disc double, the outer part, d, being screwed to the inner, e (which is solid with the 

 slider), by the three screws c, and being maintained pressed outwards by three spiral 

 springs round the screws and compressed between the two metal discs. The outer 

 edge of the disc of black glass is bevelled, and it is attached to the solid mount forming 

 part of the outer adjustable disc d by means of an annulus, which screws on to the 

 mount over the bevel, a disc of thin indiarubber being first laid in the mount, so that 

 when the black-glass disc is placed in position with the ground face against it and the 

 annulus is screwed on to just the right extent, the indiarubber pad between the solid 

 mount and the glass disc maintains the latter and the annulus in rigid connection 

 without straining or bending the glass to even the minutest extent ; if any occurred 

 it would at once be detected by curvature of the interference bands. Each microscope 



