A WAVE-LENGTH COMPARATOR FOR STANDARDS OF LENGTH. 5 



perfect record. The bauds cease to move the instant the screw is released, and each 

 band can thus be adjusted to the centre of the spider-lines, or of the reference spot 

 (the centre of a minute silvered ring carried by the fixed-glass surface), and left there 

 as long as one chooses. The process of counting the bands in such circumstances is a 

 perfectly easy matter. 



The instrument is composed of the following seven essential parts, the arrangement 

 of which will be more clearly understood from the accompanying reproductions of 

 photographs, fig. 1 giving the general view, and fig. 2 an enlarged view of the central 

 part ; also from figs. 3 and 4, giving the right and left end-elevations : 



Fig. 4. Left end-elevation. 



(1) The interferometer. This consists of (i) an autocollimating telescope, a, with 

 Geissler tube attachment, 6, and special micrometer eye-piece, c. Like the inter- 

 ferometer described by the author in connection with the dilatometer and elasmometer, 

 the telescope is mounted on a strong adjustable column provided with levelling tripod, 

 which in turn is mounted on a stone pedestal, d, in front of and isolated from the main 

 instrument, and continued down by means of concrete to the sandy loam foundations, 

 (ii) A dispersing apparatus, e, to select the rays chosen as illuminant, consisting of 

 either a pair of reflecting prisms, or a Hilger constant-deviation prism (shown in 

 position at e in fig. 3 and in figs. 1 and 2), mounted so as to be interchangeable 



