This book is a reprinting, with minor reVISIons and one correction, of notes originally prepared by John P. Brown from the lectures given in 1943 by the late Professor Witold Hurewicz at Brown University. They were first published in mimeographed form by Brown University in 1943, and were reissued by the Mathematics Department of The Massachusetts Institute of Technology in 1956. They are now reprinted with the permission of Brown University and of Dr. Stefan Hurewicz.