Determination of the Yard in Terms of the Wave-length of Light

Abstract

IN rendering due thanks to Mr. Sears, at the conclusion of my paper, for the facilities granted to me for the use of the interferential comparator, which I had the honour to devise and see constructed and installed at the Standards Department of the Board of Trade, in the time of the late Deputy-Warden of the Standards, Major P. A. MacMahon, nothing was further from my intention than to imply any responsibility on the part of Mr. Sears for the results communicated to the Royal Society in my paper. Indeed, he was from the first unsympathetic towards my interferential method. While in Major MacMahon's time the statutory periodic comparisons of the Imperial Standard Yard with its official copies were carried out with my interferential comparator, the interferometric portion was afterwards dismantled, and the instrument used for purely mechanical and microscopic comparisons. Much valuable time had to be spent at the outset of my work in cleaning, refitting, and readjusting the interferometric part, and by the Office of Works electricians in replacing perished electrical fittings, especially those of the thermostat, which maintains the whole comparator room at the official temperature of 62° F. and which had become positively dangerous from disuse. Fortunately, all the base-line work, the actual counting, band by band, of the interference bands (10,846 in yellow neon Ne_c light) in the one-eighth of an inch, was able to be carried out on my own interferometer, an improved copy of that at the Standards Department, in my private laboratory at Cambridge.