the spectrum because both nuclei are so massive that they remain almost unshaken by the dancing electron. Secondly, the helium nucleus has a double electric charge; this is equivalent to substituting in the vibrating system a controlling spring of twice the strength. What can be more natural than that the doubled force of the 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.[19] 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 ¹⁄₁₈₄₄th 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 300 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.