Light Waves as Standards of Length 105
The question may be asked: What is the object of making such determinations, when we know that the standard itself would not change by any amount which would vitiate any ordinary measurements ? The reply would be that, while the care taken of the standards is pretty sure to secure them from any serious accident, yet we have no means of knowing that any of these standards are not going through some slow process of change, on account of a gradual rearrangement of the molecules. Now that we have compared the meter with an invariable standard, we have the means of detecting any slow change and of correcting the standard which has been vitiated by such process. Thus it is now possible to control, by reference to the standard light waves, the standard of length. The standard light waves are not alterable; they depend on the properties of the atoms and upon the universal ether; and these are unalterable. It may be suggested that the whole solar system is moving through space, and that the properties of ether may differ in different portions of space. I would say that such a change, if it occurs, would not produce any material effect in a period of less than twenty millions of years, and by that time we shall probably have less interest in the problem.
summary
1. We find that three propositions for expressing our standard of length in terms of some invariable length in nature have been made, namely:
а) Measurement of the seconds pendulum.
б) Measurement of the earth's circumference.
c) Measurement of light* waves.
The first two, as well as the first plan proposed for carrying out the third, i. the method of the diffraction grating, have been found deficient in accuracy.
2. The second or interference method of utilizing light
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Light Waves and Their Uses
waves, while ideally simple in theory, necessitates in practice an elaborate and complicated piece of apparatus for its realization. But, notwithstanding the delicacy of the operation, it is capable of giving results of such extraordinary accuracy that, were the fundamental standard lost or destroyed, it could be replaced by this method with duplicates which could not be distinguished from the originals.