276 L. Bell — Absolute Wavelength of Light. 



source ' of error is common to all other determinations. It 

 seems, therefore, desirable to give at some length the various 

 comparisons on which the wave-length as given by my experi- 

 ments is based. Reserving for the present the actual meas- 

 urement of the gratings, which is a comparatively simple mat- 

 ter, I will therefore discuss the standards directly employed, 

 their relations to the Metre des Archives as found by various 

 comparisons, and finally the changes which have taken place in 

 those relations since they were first determined. 



The standards with which the gratings have been directly 

 compared are two double decimeters on speculum metal, desig- 

 nated respectively 8% and S a „. They were graduated and com- 

 pared by Prof. W. A. Rogers in 1885. The bar S, is 23 cm 

 long and bears near its edge the double decimeter S a j, subdi- 

 vided to centimeters. The defining lines are less than 1 fi in 

 width and beautifully sharp and distinct. S 2 is 2Y cm long and 

 is graduated in the same way, with lines of the same width. 

 Both standards are of the same speculum metal, and are of 

 very nearly the same mass, while the surfaces and graduation 

 leave little to be desired. The coefficient of expansion of 

 these bars was very thoroughly investigated by Prof. Rogers 

 and was found to be, 



17"946yU per meter per degree C. 



The absolute lengths of S a j and $% depend on long series of 

 comparisons with Prof. Rogers's bronze yard and meter R 2 and 

 steel copies thereof. Upon the relation existing between R^ 

 and the Metre des Archives depends then the absolute value 

 assigned to the wave-length of light, since the close agreement 

 of the various series of comparisons executed by Prof. Rogers 

 between R 2 and the speculum metal standards show that no 

 sensible uncertainty exists in the relations between them. 



The yard and meter R 2 is of the alloy known as Bailey's 

 metal, this being the material of the Imperial Yard and many 

 other standards. The graduations are upon platinum iridium 

 plugs, the polished faces of which are in the plane of one sur- 

 face of the bar when supported at its neutral points. The 

 relation of the meter R 2 to the Metres des Archives rests on a 

 very large number of comparisons made with two entirely in- 

 dependent secondary standards ; the copper meter designated 

 T, and the brass yard and meter designated C. S. A full ac- 

 count of these comparisons is contained in vol. xviii of the 

 Proceedings of the American Academy of Arts and Sciences. 



The meter T is on platinum plugs in a pure copper bar and 

 was traced and compared by M. Tresca in 1880, from the Con- 

 servatoire line meter No. 19, the relation of which to the Metre 

 des Archives was very exactly known. 



