, and since the Gaussian curvature of space depends on
, a knowledge of this curvature would yield us the total spatial extent of the universe. Now the value of the mean density of star-matter
has been computed by astronomers. If we accept this value of
, we obtain for the universe dimensions which are certainly too small, for they conflict with the enormous distances credited to the spiral nebulæ. It would appear, then, as though the cylindrical universe was incompatible with the facts of observation. But, as Einstein remarks, no reliance can be placed on the value of
commonly accepted by astronomers. In his own words: