The observation is very difficult because the Companion of Sirius is faint for work of this kind, and scattered light from its overpoweringly brilliant neighbour causes much trouble. However, after a year’s effort Professor Adams made satisfactory measurements, and he found a large shift as predicted. Expressing the results in the usual unit of kilometres per second, the mean of his measurements came to 19, whilst the predicted shift was 20. Professor Adams has thus killed two birds with one stone. He has carried out a new test of Einstein’s general theory of relativity, and he has shown that matter at least 2,000 times denser than platinum is not only possible but actually exists in the stellar universe.[13] This is the best confirmation we could have for our view that the sun with a density 1½ times that of water is still very far indeed from the maximum density of stellar matter; and it is therefore entirely reasonable that we should find it behaving like a perfect gas. I have said that the observation was exceedingly difficult. However experienced the observer, I do not think we ought to put implicit trust in a result which strains his skill to the utmost until it has been verified by others working independently. Therefore you should for the present make the usual reservations in accepting these conclusions. But science is not just a catalogue of ascertained facts about the universe; it is a mode of progress, sometimes tortuous, sometimes uncertain. And our interest in science is not merely a desire to hear the latest facts added to the collection; we like to discuss our hopes and fears, probabilities and expectations. I have told the detective story so far as it has yet unrolled itself. I do not know whether we have reached the last chapter. _Unknown Atoms and Interpretation of Spectra_ It should be understood that this matter of enormous density is not supposed to be any strange substance--a new chemical element or elements. It is just ordinary matter smashed about by the high temperature and so capable of being packed more tightly--just as more people could be squeezed into a room if a few bones were broken. It is one of the features of astronomical physics that it shows us the _ordinary_ elements of the earth in an _extraordinary_ state--smashed or ionized to a degree that has either not been reproduced or has been reproduced with great difficulty in the laboratory. It is not only in the inaccessible interior of the star that we find matter in a state outside terrestrial experience.