This seems to be the beginning of a great discovery, but it ends in a bathos. It happens that we were already aware of a medium containing oxygen lying somewhere between our telescope and the sun. It is a medium essential to our existence. The terrestrial atmosphere is responsible for the ‘fixed’ lines seen in the sun’s spectrum. Just as the spectroscope can tell us that the sun is turning round (a fact already familiar to us from watching the surface markings), so it can tell us that certain stars are wandering round an orbit, and therefore are under the influence of a second star which may or may not be visible itself. But here again we sometimes find ‘fixed’ lines which do not change with the others. Therefore somewhere between the star and the telescope there exists a stationary medium which imprints these lines on the light. This time it is not the earth’s atmosphere. The lines belong to two elements, calcium and sodium, neither of which occur in the atmosphere. Moreover, the calcium is in a smashed state, having lost one of its electrons, and the conditions in our atmosphere are not such as would cause this loss. There seems to be no doubt that the medium containing the sodium and ionized calcium--and no doubt many other elements which do not show themselves--is separate from the earth and the star. It is the ‘fullness’ of interstellar space already mentioned. Light has to pass one atom per cubic inch all the way from the star to the earth, and it will pass quite enough atoms during its journey of many hundred billion miles to imprint these dark lines on its spectrum.