measured relatively to other stars, preferably more distant from the sun and less displaced; we need therefore a reasonable number of outer bright stars to serve as reference points.

In a superstitious age a natural philosopher wishing to perform an important experiment would consult an astrologer to ascertain an auspicious moment for the trial. With better reason, an astronomer to-day consulting the stars would announce that the most favourable day of the year for weighing light is May 29. The reason is that the sun in its annual journey round the ecliptic goes through fields of stars of varying richness, but on May 29 it is in the midst of a quite exceptional patch of bright stars part of the Hyades by far the best star-field encountered. Now if this problem had been put forward at some other period of history, it might have been necessary to wait some thousands of years for a total eclipse of the sun to happen on the lucky date. But by strange good fortune an eclipse did happen on May 29, 1919. Owing to the curious sequence of eclipses a similar opportunity will recur in 1938; we are in the midst of the most favourable cycle. It is not suggested that it is impossible to make the test at other eclipses, but the work will necessarily be more difficult.

Attention was called to this remarkable opportunity by the Astronomer Royal in March, 1917; and preparations were begun by a Committee of the Royal Society and Royal Astronomical Society for making the observations. Two expeditions were sent to different places on the line of totality to minimise the risk of failure by bad weather. Dr A. C. D. Crommelinand Mr C. Davidson went to Sobral in North Brazil; Mr E. T. Cottingham and the writer went to the Isle of Principe in the Gulf of Guinea, West Africa....

It will be remembered that Einstein's theory predicts a deflection of 1"-74 at the edge of the sun*, the amount falling off inversely as the distance from the sun's centre. The simple Newtonian deflection is half this, o"-87. The final results (reduced to the edge of the sun) obtained at Sobral and Principe with their "probable accidental errors" were

Sobral 1"-98±0.12

Principe 1".61±0.30.

The predicted deflection of light from infinity to infinity is just over 1'745; from infinity to the earth it is just under.

It is usual to allow a margin of safety of about twice the probable error on either side of the mean. The evidence of the Principe plates is thus just about sufficient to rule out the possibility of the "half-deflection," and the Sobral plates exclude it with practical certainty. The value of the material found at Principe cannot be put higher than about one-sixth of that at Sobral; but it certainly makes it less easy to bring criticism against this confirmation of Einstein's theory seeing that it was obtained independently with two different instruments at different places and with different kinds of checks.

The best check on the results obtained with the 4-inch lens at Sobral is the striking internal accordance of the measures for different stars. The theoretical deflection should vary inversely as the distance from the sun's centre; hence, if we plot the mean radial displacement found for each star separately against the inverse distance, the points should lie on a straight line. This is shown in Fig. 17 where the broken line shows the theoretical prediction of Einstein, the deviations being within the accidental errors of the determinations. A line of half the slope representing the half-deflection would clearly be inadmissible....

We have seen that the swift-moving light-waves possess great advantages as a means of exploring the non-Euclidean property of space. But there is an old fable about the hare and the tortoise. The slow-moving planets have qualities which must not be overlooked. The light-wave traverses the region in a few minutes and makes its report; the planet plods on and on for centuries, going over the same ground again and again. Each time it goes round it reveals a little about the space, and the knowledge slowly accumulates.

According to Newton's law a planet moves round the sun in an ellipse, and if there are no other planets disturbing it, the ellipse remains the same for ever. According to Einstein's law the path is very nearly an ellipse, but it does not quite close up; and in the next revolution the path has advanced slightly in the same direction as that in which the planet was moving. The orbit is thus an ellipse which very slowly revolves*.

The exact prediction of Einstein's law is that in one revolution of the planet the orbit will advance through a fraction of a revolution equal to 3v2/C2, where v is the speed of the planet

* Appendix, Note 9.

on

and C the speed of light. The earth has 1/10,000 of the speed of light; thus in one revolution (one year) the point where the earth is at greatest distance from the sun will move 3/100,000,000 of a revolution, or o"-038. We could not detect this difference in a year, but we can let it add up for a century at least. It would then be observable but for one thing-the Displacement

earth's orbit is very blunt, very nearly circular, and so we cannot tell accurately enough which way it is pointing and how its sharpest axes move. We can choose a planet with higher speed so that the effect is increased, not only because 2 is increased, but because the revolutions take less time; but, what is perhaps more important, we need a planet with a sharp elliptical orbit, so that it is easy to observe how its apses move round. Both

these conditions are fulfilled in the case of Mercury. It is the fastest of the planets, and the predicted advance of the orbit amounts to 43" per century; further the eccentricity of its orbit is far greater than of any of the other seven planets.

Now an unexplained advance of the orbit of Mercury had long been known. It had occupied the attention of Le Verrier, who, having successfully predicted the planet Neptune from the disturbances of Uranus, thought that the anomalous motion of Mercury might be due to an interior planet, which was called Vulcan in anticipation. But, though thoroughly sought for, Vulcan has never turned up. Shortly before Einstein arrived at his law of gravitation, the accepted figures were as follows. The actual observed advance of the orbit was 574" per century; the calculated perturbations produced by all the known planets amounted to 532" per century. The excess of 42′′ per century remained to be explained. Although the amount could scarcely be relied on to a second of arc, it was at least thirty times as great as the probable accidental error.

The big discrepancy from the Newtonian gravitational theory is thus in agreement with Einstein's prediction of an advance of 43′′ per century..........

The theory of relativity has passed in review the whole subject-matter of physics. It has unified the great laws, which by the precision of their formulation and the exactness of their application have won the proud place in human knowledge which physical science holds to-day. And yet, in regard to the nature of things, this knowledge is only an empty shell—a form of symbols. It is knowledge of structural form, and not knowledge of content. All through the physical world runs that unknown content, which must surely be the stuff of our consciousness. Here is a hint of aspects deep within the world of physics, and yet unattainable by the methods of physics. And, moreover, we have found that where science has progressed the farthest, the mind has but regained from nature that which the mind has put into nature.

We have found a strange foot-print on the shores of the unknown. We have devised profound theories, one after another, to account for its origin. At last, we have succeeded in reconstructing the creature that made the foot-print. And Lo! it is our own.

II. THE ATOMIC THEORY

LUCRETIUS

69

THE second great problem which faces the enquirer into Nature is the structure of matter. Is matter continuous? Can it be subdivided without limit? Is water water and iron iron, if a drop of the one or a lump of the other be cut in half an infinite number of times? Or shall we come eventually to particles which, could they be still further divided, would give smaller particles no longer of water or iron but of some more fundamental stuff of which perhaps both water and iron are made?

As far as historic records go, this problem was first attacked in a systematic way by the early Ionian philosophers. They speculated about a single element, a common basis to all substances. On the other hand, Empedocles taught four primary elements, earth, water, air and fire. Leucippus and Democritus developed these concepts into a definite atomic theory, known to us best by the account of it given by the Latin Poet Lucretius.

The true meaning of the theory was its attempt to explain the different qualities of bodies in terms of rational ideas. "According to convention," says Democritus, "there is a sweet and a bitter, a hot and a cold, and according to convention there is colour. In truth there are atoms and a void." By differences in the size, shape, position and movement of atoms identical in substance, all the various kinds of matter are formed. This attempted explanation of facts by apparently simpler ideas is the essence of scientific advance. We now see that the theory of the atomists does but push back the mystery one step, while leaving it unsolved. But when such a step is made, the mind of man inevitably overestimates its importance and a story curiously reiterated through history-discards the gods who dwell behind the mystery. The atomists of the ancient world were attacked as atheists. The French encyclopædists of the eighteenth century thought that they were not far from an explanation of the universe, and Laplace told Napoleon he had no need of the "hypothesis of God." The Inquisition condemned Galileo, the Bishops repudiated Darwin, and Huxley led a counter attack on the Bishops. Neither side has hurt the other, since their true domains are on different planes. But both have had to give up some untenable ground they had no right to occupy. To illustrate the atomic theory of the Greeks, we give some extracts from a translation of Lucretius made by John Evelyn, the famous diarist and author of Evelyn's Sylva. Our extracts from the first book of the poem proclaim that "nothing from nothing comes," i.e. all things have an origin and matter is indestructible. It may seem to vanish, but it exists as minute insensible "seeds of all things" or atoms.