[15] for a wave-length of 400
with certainty.
= Ångström unit = 10-8 cm.
For the conditions of emission of the light from the sun's surface have not yet been sufficiently investigated, and the systematic errors in the wave-lengths in the light from the source used for comparison on the earth, the arc-lamp, are not yet sufficiently known to allow the negative results of observation hitherto obtained to be regarded as giving binding decisions. This is the more true inasmuch as in the case of the fixed stars there are, doubtless, signs of the presence of a gravitational shift of the spectral lines (vide the closing essay [The Third Test] of this book). It is a particularly important task of astronomy to establish this effect with certainty, for this gravitational displacement of the spectral lines is a direct consequence of the hypothesis of equivalence, and does not assume the other hypotheses of the theory such as, for example, the differential equations of the gravitational field.
The third and particularly important inference from Einstein's theory is the dependence of the velocity of light upon the gravitational potential, and the resultant curvature (based upon Huygens' principle) of a ray of light in passing through a gravitational field. The theory asserts that a ray of light, coming e.g. from a fixed star, and which passes in close proximity to the sun, has a curved path. As a consequence of this curvature, the star must appear displaced from its true position in the heavens by an amount which attains the value 1.7" at the edge of the sun's disc, and decreases in proportion to the distance from the centre of the sun. But since a ray of light which comes from a fixed star and passes by the sun can be caught only when the light of the sun, which overpowers all else by its brilliancy, is intercepted before its entrance into our atmosphere, only the rare moments of a total eclipse come into account for this observation and for the solution of the problem. The solar eclipse of 29th May, 1919, during which photographs were taken at two widely-separated stations, for the purpose of this test, has, as far as the results of measurement allow us to pass definite judgment, decided in favour of the general theory of relativity.[16]
The experimental verification of Einstein's theory of gravitation has thus not reached completion. But if, in spite of this, the theory can, even at this early stage, justly claim general attention, the reason is to be found in the unusual unity and logical structure of the ideas underlying it. In truth, it solves, at one stroke, all the riddles, concerning the motions of bodies, which have presented themselves since the time of Newton, as the result of the conventional view about the meaning of space and time in the physical description of natural phenomena.