It should be understood that it was very difficult to explain away the original message as a mistake. As to the mass being ⅘ths of the sun’s mass there can be no serious doubt at all. It is one of the very best determinations of stellar mass. Moreover, it is obvious that the mass must be large if it is to sway Sirius out of its course and upset its punctuality as a clock. The determination of the radius is less direct, but it is made by a method which has had conspicuous success when applied to other stars. For example, the radius of the huge star Betelgeuse was first calculated in this way; afterwards it was found possible to measure directly the radius of Betelgeuse by means of an interferometer devised by Michelson, and the direct measurement confirmed the calculated value. Again the Companion of Sirius does not stand alone in its peculiarity. At least two other stars have sent us messages proclaiming incredibly high density; and considering our very limited opportunities for detecting this condition, there can be little doubt that these ‘white dwarfs’, as they are called, are comparatively abundant in the stellar universe. But we do not want to trust entirely to one clue lest it prove false in some unsuspected way. Therefore in 1924 Professor Adams set to work again to apply to the message a test which ought to be crucial. Einstein’s theory of gravitation indicates that all the lines of the spectrum of a star will be slightly displaced towards the red end of the spectrum as compared with the corresponding terrestrial lines. On the sun the effect is almost too small to be detected having regard to the many causes of slight shift which have to be disentangled. To me personally Einstein’s theory gives much stronger assurance of the real existence of the effect than does the observational evidence available. Still it is a striking fact that those who have made the investigation are now unanimous in their judgement that the effect really occurs on the sun, although some of them at first thought that they had evidence against it. Hitherto Einstein’s theory has been chiefly regarded by the practical astronomer as something he is asked to test; but now the theory has a chance to show its mettle by helping us to test something much more doubtful than itself. The Einstein effect is proportional to the mass divided by the radius of the star; and since the radius of the Companion of Sirius is very small (if the message is right) the effect will be very large. It should in fact be thirty times as large as on the sun. That lifts it much above all the secondary causes of shift of the lines which made the test on the sun so uncertain.