THE GREAT STAR MAP. 247 



infer its existence from the fact that the street lamps seemed 

 only to extend to a certain distance, instead of being visible 

 indefinitely, 



A closely similar explanation can be given of the appearance 

 of centrality suggested by the star counts : the universe may 

 be filled with a slight fog. It must, of course, be so extremely 

 tenuous that the name fog is completely unsuitable ; for that 

 name suggests to us something which quenches light very 

 rapidly, so that within a few yards (sometimes within a few 

 inches) the brightness of a light would be reduced to one-half. 

 The " fog" in space must require at least thousands of billions 

 of miles to effect the same reduction to one-half The size of 

 these figures does not mean that they are hopelessly vague : 

 indeed we are almost in a position to say that the number of 

 thousands of billions must be greater than 4 and less than 40, 

 for various independent discussions of this most important 

 matter have been made recently, and they all point to figures 

 within the limits just specified. 



From the star counts alone we could not infer the existence 

 of this light-extinguishing medium, which we may continue to 

 call a " fog " for brevity. At any rate the alternative of a 

 limited universe would have equal claims to consideration. 

 But the evidence for the fog has been steadily growing. In 

 the first place we have had before our eyes for centuries the 

 spectacle of finely divided matter being driven off into space — 

 in comets' tails and in the sun's corona. There are various 

 interpretations of both these phenomena but the facts cannot 

 be accounted for completely without some hypothesis of the 

 escape of matter into space. Again, it has been realised that 

 particles must be continually escaping from planetary atmo- 

 spheressuch as our own. Thereis,in fact,nodoubt of the existence 

 of matter in the spaces between the stars : the only question 

 is as to its amount. And, as a second line of evidence, the 

 spectroscope seems to indicate that the amount is appreciable. 

 Professor Newall of Cambridge was so much impressed with 

 the accumulated evidence of the spectroscope that he devoted 

 to the subject a special Presidential Address to the Royal 

 Astronomical Society in February 1909. " Here, then," he 

 summarised, " are a few reasons for looking into possible 

 practical ways of justifying the belief that in space, especially 

 in the neighbourhood of suns, there must exist matter forming 



