This displacement to the red as a result of mere distance is peculiar to de Sitter’s cosmology. It is additional to the displacement which, as all cosmologies agree, the spectrum of a moving body must shew as the result of its motion, this latter being towards the red only if the body is receding from the earth (p. 51). On de Sitter’s cosmology, the two displacements are not entirely independent, for it is an essential feature of this cosmology that near bodies should tend to move further apart from one another. Just as bits of straw thrown together into a stream tend to get separated as they float down the stream, so objects in de Sitter’s universe move further apart as they float down the stream of time. Thus on de Sitter’s theory a displacement of spectral lines to the red cannot be interpreted as evidence either of motion or of distance; it is a mixture of both. This does not mean that we have been altogether wrong in deducing the velocities of stars in the galactic system from the observed displacements of their spectral lines. No appreciable displacement is produced by distance alone, unless this distance forms an appreciable fraction of the radius of the universe. Systematic displacements to the red are, it is true, observed in the spectra of the most distant stars, but they are of very small amount. It is only when we look to the remote extra-galactic nebulae that we can expect to observe the effect in appreciable strength. Now it has long been one of the outstanding puzzles of astronomy that the spectra of the distant nebulae are uniformly displaced towards the red. The observed displacements are not small. Interpreted as velocities, many of them would represent speeds of over 1000 miles a second. Humason has found that two faint nebulae N.G.C. 4860 and 4853 shew apparent speeds of recession of 4900 and 4600 miles a second respectively. They are both members of a cloud of nebulae in Coma Berenices (p. 72), which is probably at a distance of about 50 million light-years. Quite recently the spectrum of another very faint nebula in Ursa Major has been found to indicate an apparent motion of recession with a speed of 7200 miles a second. If de Sitter’s theory is rejected, almost all the extra-galactic nebulae must be running away from us with terrific, almost unimaginable, speeds. And the further away they are the more precipitately they are increasing their distance. Yet we can hardly reintroduce simplicity by adopting de Sitter’s theory, and treating the whole apparent stampede of nebulae as spurious, since this theory involves that the nebulae may well, in actual fact, be running away from us, scattering being an inherent property of objects in a de Sitter universe.