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.

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 [Note: My references to 'perfect gas of the density of platinum' and 'material 2,000 times denser than platinum' have often been run together by reporters into 'perfect gas 2,000 times denser than platinum'. It is scarcely possible to calculate what is the condition of the material in the Companion of Sirius, but I do not expect it to be a perfect gas.] is not only possible but actually exists in the stellar universe. This is the best confirmation we could have for our view that the sun with a density 1 1/2 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 difficultly in the laboratory. It is not only in the inaccessible interior of the star that we find matter in a state outside terrestrial experience.

• Fig 8. The Ring Nebula in Lyra. Slitless Spectrogram.
• Photographed by Dr. W. H. Wright at the Lick Observatory, California.

Here is a picture of the Ring Nebula in Lyra (Fig. 8). It is taken through a prism so that we see not one ring but a number of rings corresponding to different lines of the spectrum and representing the different kinds of atoms which are at work producing the light of the nebula. The smallest ring, which is rather faint (marked by an arrow), consists of light produced by the helium atoms in the nebula -- not ordinary