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[16] By means of the star photographs of two expeditions equipped

by a Joint Committee of the Royal and Royal Astronomical

Societies, the existence of the deflection of light demanded by

theory was first confirmed during the solar eclipse of 29th May,

1919. (Cf. Appendix III.)

In the second place our result shows that, according to the

general theory of relativity, the law of the constancy of the

velocity of light in vacuo, which constitutes one of the two

fundamental assumptions in the special theory of relativity and

to which we have already frequently referred, cannot claim any

unlimited validity. A curvature of rays of light can only take

place when the velocity of propagation of light varies with

position. Now we might think that as a consequence of this, the

special theory of relativity and with it the whole theory of

relativity would be laid in the dust. But in reality this is not

the case. We can only conclude that the special theory of

relativity cannot claim an unlimited domain of validity; its

results hold only so long as we are able to disregard the

influences of gravitational fields on the phenomena (e.g. of

light).