12 
The Astronomical Survey of the Universe [oh. i 
Clearly the “period-luminosity” law provides a powerful method for the 
determination of astronomical distances; like the method of spectroscopic 
parallaxes, it is especially valuable because it supplements the parallactic 
method just where the latter fails. However distant Cepheid variables may 
be, provided only they can be clearly seen in a telescope, the astronomer can 
pick them out from the main mass of stars by their regular and characteristic 
light-fluctuations with the same ease and certainty with which the mariner 
picks out a lighthouse from a confusion of other lights on shore. The subse 
quent procedure is precisely that of the mariner who, having picked up a 
lighthouse, looks up its candle-power on a chart and estimates his distance 
from it by comparing its known candle-power with its apparent brightness ; 
the “period-luminosity” law gives us the candle-powers of the Cepheid 
variables. The corresponding analogy with the parallactic method would be 
if the mariner, knowing the speed of his ship, estimated his distance from 
land by noticing the rate at which an electric standard or other fixed light 
on the sea-front appeared to move against a distant background of lights. 
The latter method is independent of the existence of lighthouses of known 
candle-power, but is obviously useless for ships far out at sea. 
12. In 1918 Shapley used the “period-luminosity” law to determine the 
distances of the globular clusters, and found these to range from 22,000 to 
220,000 light-years. At such distances the parallactic method would fail 
hopelessly: the parallactic orbit of a star at 220,000 light-years distance 
would have the same apparent size as a pin-head at 3000 miles. This bald 
statement gives but little real conception of the remoteness of the star-clusters. 
Their distance is perhaps better conveyed by the reflection that the light by 
which we now see them left them about the time when primaeval man first 
appeared on earth. Through the childhood, youth and age of countless 
generations of men, through the long prehistoric ages, through the slow dawn 
of civilisation and through the whole span of time which history records, this 
light has travelled steadily on its course, covering 186,000 miles every second, 
and is only reaching us now. 
Even the distances first mentioned are not the greatest at which Cepheid 
variables are visible, for in 1924 Dr Hubble of Mount Wilson, detected them 
in the nearer spiral nebulae, and so was able to shew that the distances of 
these spirals is of the order of a million light-years, or nearly five times that 
of the remotest of the star-clusters. Using these nearer spiral nebulae as 
stepping-stones, he has since found that the remotest of the spiral nebulae 
which are visible must be over a hundred times as distant as the nearest, 
bringing us to distances of the order of 140 million light-years (cf. § 17 below). 
The Distribution of Stars in Space. 
13 . It is natural to wonder how these vast ranges of space are filled. In 
the neighbourhood of the sun we have found that stars are uniformly scattered