ing accurately the whole number of waves in the last standard. The whole number obtained by this process of "stepping off" for the red radiation of cadmium was found to be 310,678. The fraction was then determined by the circular fringes, as described above, and found to be .48. In the same way the number for the green radiation was determined as 393,307.93; and for the blue radiation as 416,735.86. To give an idea of the order of accuracy, I would state that there were three separate determinations made at different times and by different individuals, as follows:
The fact that these determinations were made at entirely different times, separated by an interval of whole months, and by different individuals, and that we still were able to get, not only the same whole number of waves, but also so nearly the same fractions, gives us a confidence, which we could not otherwise feel, in the possibilities of the process.
In comparing the standards with one another the temperature made no difference, if only it were uniform throughout the instrument, because two intermediate standards side by side, made of the same substance, would expand in exactly the same way, provided we could be sure that both had the same temperature. But in the determination of the number of waves in standard No. 9 it is extremely important to know the temperature with the very highest degree of accuracy. For this purpose some of the best thermome-
