PHILOSOPHICAL TRANSACTIONS. 



I. A Wave-Length Comparator for Standards of Length : An Instrument for 

 Fine Measurement in Wave-Lengths of Light. 



With an Appendix on the Use of Wave-Length Rulings as Defining Lines on 



Standards of Length. 



By A. E. H. TUTTON, M.A., D.Sc., F.R.S. 

 Received June 9, Read June 10, 1909. 



THE following is a brief account of a new apparatus for fine measurement in wave- 

 lengths of light, designed primarily as a comparator for the measurement in wave- 

 lengths of the difference between a standard of length, either a line or an end 

 measure bar the Imperial Standard Yard, for instance and any duplicate or 

 similar bar proposed to be employed as a derived standard. The instrument is 

 also, however, the most perfect instrument yet devised for measurement in wave- 

 lengths in general, and performs its functions so admirably as to render it highly 

 desirable that a description should now be published concerning it. It has been 

 constructed to the designs and under the supervision of the author for the Standards 

 Department of the Board of Trade, and this account of it is communicated to the 

 Royal Society with the permission of the President of the Board of Trade. The 

 principle underlying the instrument is that of the author's interferometer,* which has 

 also proved so successful in its application, in the interference dilatometer,t to the 

 determination of the thermal expansion of small bodies by the Fizeau method, and in the 

 elasmometerj to the measurement of the elastic bending of a small plate or bar under 

 a given weight applied at the centre. The essence of the interferometer is that 

 homogeneous light, of a definite wave-length, corresponding to a single spectrum line 

 isolated with the aid of a constant-deviation prism from the spectrum derived from 

 a cadmium or hydrogen Geissler tube, or a mercury lamp is directed by an auto- 

 collimation method, ensuring identity of path of the incident and reflected rays, 

 normally upon two absolutely plane surfaces, arranged close to each other, and nearly, 

 but not absolutely, parallel ; the two reflected rays give rise, by their interference, to 



* ' Phil. Trans.,' A, 1898, vol. 191, p. 324. 



t 'Phil. Trans.,' A, 1898, vol. 191, p. 313. 



| ' Phil. Trans.,' A, 1904, vol. 202, p. 143. 



VOL. CCX. A 459. B 28.10.09 



