The two books considered here, despite the romantic title of the larger and more ambitious one, are complementary both in subject matter and in interest. In The Sleepwalkers, Arthur Koestler offers us a highly un-conventional, and in detail controversial, history of man's picture of the Cosmos. But he brings it to a close with the Newtonian synthesis since this afforded a simple scheme satisfying to mathematician and layman alike. This, he claims, is the picture still accepted by the majority today, and as regards the ‘man in the street’ and the school textbook he may be correct. But during the present century there has been an accelerating series of amazing astronomical discoveries which in their earlier stages were excitingly presented to the reading public by Eddington and Jeans. Last year Professor Lovell, in the Reith Lectures, brought listeners up to date, and dealt briefly with the individual man's place within the vast dimensions now revealed.