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 light from the two limbs of the sun is the same. The possibility that an original difference in velocity would be destroyed when the light reached the neighborhood of the earth is not entirely excluded. Nevertheless, the experiments of Fizeau and Michelson which we have just discussed seem to show that the presence of air or other transmitting medium would not completely destroy such a difference. Furthermore, the experiment of Sir Oliver Lodge has led us to expect no change in the velocity of light produced by the neighborhood of large masses such as the earth. It may also be pointed out in this connection, that, up to the present time, no astronomical data of any kind have been found which are in disagreement with the principle that the velocity of light is independent of that of the source. For example, it has been shown by Comstock that a difference in the velocity of light from approaching and receding stars would lead us to expect irregularities in the observed orbital motion of double stars which have never been detected. Certainly, until further evidence is presented, we may best accept that principle regarding the velocity of light which has led to the second postulate of relativity.

We shall now consider an entirely different method of proving the second postulate of relativity.

The Kaufmann-Bucherer Experiment.
Certain conclusions of the theory of relativity have been quantitatively verified by the experiments of Kaufmann and of Bucherer on the mass of the β particle. It has already been stated in the article of Lewis and Tolman referred to above that this experimental fact may itself be used for the reverse process of deducing the second postulate, and a method of proof was worked out at the time that paper was published. It is very desirable to consider this proof since it includes a deduction, without the help of the second postulate, of all the changes in the units of length and time, to which the theory of relativity has led, and finally gives a proof of the second postulate itself.

Let us suppose an electron ε at rest and an electron ε'  moving past it with the velocity v.