spring should double the number of lines in the series without otherwise altering its plan? In this way Professor Bohr discovered the real origin of the Pickering Series; it is due to ionized helium, not to hydrogen. [Note: The helium line in the Ring Nebula on which we have already commented is not a member of the Pickering Series, but it has had the same history. It was first supposed to be due to hydrogen, later (in 1912) reproduced by Fowler terrestrially in a mixture of helium and hydrogen, and finally discovered by Bohr to belong to helium.]

The heavy nucleus, whether of hydrogen or helium, remains almost unshaken by the atomic vibration almost, but not quite. At a later date Professor A. Fowler succeeded in reproducing the Pickering Series in the laboratory and was able to measure the lines with much greater accuracy than could be achieved in stellar spectroscopy; he was then able to show from his measures that the nucleus is not quite irresponsive. It was a delicate double-star problem transferred to the interior of the atom; or perhaps a closer analogy would be the mutual influence of the sun and Jupiter, because Jupiter, having a thousandth of the mass of the sun, disturbs it to about the same extent that the light electron disturbs the hydrogen nucleus. Ionized helium is a faithful copy of the hydrogen atom (on the altered scale) in everything except the 'shake'; the shake is less than in hydrogen because the helium nucleus is still more massive and rock- like. The difference of shake throws the Pickering Series of helium and the Balmer Series of hydrogen slightly out of step with respect to one another; and by measuring this misfit Professor Fowler was able to make a very accurate determination of the shake and therefore of the mass of the electron. In this way the mass of the electron is found to be 1/1,844th of the mass of the hydrogen nucleus; this agrees well with the mass found by other methods, and the determination is probably not inferior in accuracy to any of them.

And so the clue first picked up in stars 500 light years away, followed in turn by the theoretical and the experimental physicist, leads in the end to the smallest of all things known.

The Cloud in Space

Having already considered the densest matter in the universe, we now turn to consider the rarest.

In spite of great improvements in the art of exhausting vessels we are still a long way from producing a real vacuum. The atoms in a vacuum tube before it is exhausted muster a formidable number containing about twenty digits. High exhaustion means knocking off five or six noughts at the end of that number; and the most strenuous efforts to knock off one more nought seem ludicrously ineffective -- a mere nibbling at the huge number that must remain.

Some of the stars are extremely rarefied. Betelgeuse, for example, has a density about a thousandth that of air. We should call it a vacuum were it not contrasted with the much greater vacuosity of surrounding space. Nowadays physicists have no difficulty in producing a better vacuum than Betelgeuse; but in earlier times this star would have been regarded as a very creditable attempt at a vacuum.

The outer parts of a star, and especially the light appendages such as the solar chromosphere and corona, reach much lower densities. Also the gaseous nebulae are, as their appearance suggests, extremely tenuous. When there is space enough to put a pin's head between adjacent atoms we can begin to talk about a 'real vacuum.' At the centre of the Orion nebula that degree of rarefaction is probably reached and surpassed.

A nebula has no definite boundary and the density gradually fades off. There is reason to think that the fading off becomes slow at great distances. Before we pass entirely out of the sphere of one nebula we enter the sphere of another, so that there is always some residual density in interstellar space.

I believe that, reasoning from the tailing off of the nebulae, we are in a position to make an estimate of the amount of matter remaining unaggregated in space. An ordinary region where there is no observable nebulosity is the highest vacuum existing -- within the limits of the stellar system at least -- but there still remains about one atom in every cubic inch. It depends on our point of view whether we regard this