January 20, 1910] 



NA TURE 



339 



his interferometer to the purposes of a wave-length 

 xomparator of measures of length, and a memoir 

 recently published in the Philosophical Transactions 

 of the Royal Society (Phil. Trans. A, 1910, 

 vol. ccx., pi 1), with the consent of the. Presi- 

 dent of the Board of Trade, describes the instrument, 

 which has now been installed in the Standards Office. 

 The memoir also comprises an appendix concerning 

 the possible employment of wave-length rulings on 

 metal as defining lines on standard bars, with sug- 

 gestions for their use along with the interference 

 bands of the interferometer, in an original method of 

 determining the total number of wave-lengths in the 

 British yard. 



.\ general view of the interferometer and one of 

 the duplicate microscopes of the comparator, together 

 v.ith sufficient of the bar-carriage to enable some idea 



the interfering light ; the rays from the Geissler tube, 

 received on the other face of the right-angled prism, 

 are arranged to fill this stop after reflection from the 

 hypotenuse of the prism. The rays proceed from the 

 slop to the objective, which they are arranged to fill 

 with light, and thence pass out of the telescope as 

 parallel rays, in the path of which the dispersion and 

 interference apparatus is placed. The rays return 

 to the telescope from the latter along practically the 

 same path, but after re-entering the telescope, instead 

 of returning to the little rectangular stop, their origin, 

 they are deflected just sufficiently to one side to 

 form an image of the stop, the same size as the 

 original, in the open semicircular aperture of the 

 focal plane, within a couple of millimetres of the 

 real stop. This closeness to identity of path of the 

 outgoing and incoming rays, and consequent normal 



Central part of comparator, shovving infercD 



of the whole apparatus to be gained, is given in the 

 accompanying illustration. 



The whole instrument is mounted on a large stone 

 block, resting on isolated concrete foundations. On 

 a small stone pedestal, similarly isolated, in front 

 of the large block, rests the pedestal of the auto- 

 collimating telescope and attached Geissler tube of 

 the interferometer. In the common focal plane of the 

 telescope objective and eye-piece, opposite the junction 

 of this main optical tube with the rectangularly 

 attached side-tube carrying the Geissler tube, a small 

 totally reflecting prism is arranged, half covering the 

 focal aperture. A still smaller rectangular stop or 

 opening in a plate in front of and almost touching 

 that one of the perpendicular prism faces which is 

 directed towards the objective, and lies in the 

 focal plane very close to the edge, dividing the closed 

 half from the open half, is the efTective source of 

 NO. 2099, VOL. 82] 



incidence on the reflecting glass surfaces of the inter- 

 ference apparatus, is largely responsible for the mag- 

 nificent field of parallel straight-lined interference 

 bands which the author's interferometer affords, for it 

 fulfils an essential condition for perfect interference. 



With the ordinary eye-piece in position, the images 

 of the stop reflected from the various surfaces of the 

 interference apparatus can be focussed, adequately 

 magnified, and viewed during their adjustment to the 

 theoretically ideal positions. But when this eye-piece 

 is replaced by a special one consisting of a Ramsden 

 micrometer combined with an additional lens between 

 the latter and the focal plane, the telescope is con- 

 verted into a low-power microscope, which focusses 

 simultaneously the interference bands, a little silvered 

 reference ring in the centre of one of the two 

 surfaces reflecting the interfering light, and the 

 micrometer spider-lines. There are two parallel 



