A WAVE-LENGTH COMPARATOR FOR STANDARDS OF LENGTH. 



11 



elasmometer, except that there are added at the base of the pedestal, above the 

 tripod, two solidly constructed rectangular movements, a and ft, for the adjustment 

 of the pedestal in the right-and-left and front-and-back directions in the horizontal 

 plane. These movements are a great convenience in placing the circular disc of 

 interference b.inds, that is, the image of the black-glass reflecting surface, g s , crossed 

 by the bands, centrally in the field of view ; and also for adjusting the telescope so as 

 to observe the image in light of different wave-lengths, when employing the pair of 

 refraction prisms as dispersing apparatus. The "comb" or rough scale of the 

 micrometer of the eye-piece is also different, being made adjustable to different heights 

 in the field of view by vertical sliding in a slot, the slider projecting to the necessary 

 slight extent outside (seen at y, fig. 3), which enables it to be altogether pushed 

 out of the field if desired, leaving the complete circular disc of light vertically crossed 

 by interference bands fully visible. 



The actual appearance of the field of 

 interference bands and of the comb is 

 shown in fig. 6. The silvered reference 

 ring occupies the centre of the field, with 

 a band adjusted to its centre, and the 

 spider-lines adjusted on each side of the 

 band. 



Provision is made for electrically heat- 

 ing the outer limb of the cadmium 

 vacuum tube, when such is employed 

 instead of a hydrogen tube, by means of 

 a spiral of platinum wire embedded 

 between two jackets of asbestos, the 

 whole being enclosed in an aluminium 

 tube. The arrangement is seen along 

 with the cadmium tube resting on a 

 mahogany tray at the base of the interferometer in fig. 1. When the tube and 

 fitting are in position instead of the hydrogen tube shown in situ in the figures, on 

 switching on the current, suitably resisted down to a safe amount by filtration 

 through a special resistance lamp, the platinum spiral becomes sufficiently heated to 

 raise this limb of the tube, which contains a pellet of cadmium, to between 200 and 

 300 C., when the cadmium spectrum is intensely generated. 



With respect to the dispersion apparatus referred to under (1, ii), the use of a 

 Hilger constant-deviation prism, as an alternative to the two refracting prisms 

 employed on the dilatometer, is an innovation. As the plane of the axis of the whole 

 optical arrangement, including the dispersion apparatus, is now horizontal, a constant 

 deviation prism can conveniently be employed and proves very efficient for the 

 purpose ; and although the dispersion is not so great as with the two refracting 



C 2 



Fig. 6. Field of interference bands. 



