For uncounted generations thoughtful men, philosophers and mathematicians, physicists and astronomers, as well as many other students, have been puzzling over the nature of our surroundings in space. They have not been satisfied, as I am afraid many of us are satisfied, with their merely terrestrial surroundings but have tiied to penetrate the almost inconceivable distancee that separate the members of our stellar system, to trace the evolution and unravel some of the mysteries of the structure and constitution of the universe. The word universe, however, is so broad and inclusive in its meaning, it implies something so vast and incomprehensible to our finite minds, as to be quite beyond the possibility of useful definition or discussion in a lecture like this. The term, universe, has frequently been used, and indeed is still used, I believe improperly, as 'referring to the particular system of stars of which our Sun is one insignificant member among thousands of millions of others. A much better designation, however, is "galaxy" and hereafter we will refer to the system in which we are situated as the Galaxy. Although it is now almost universally believed that there are countless other stellar systems extending to distances beyond the capacity of the most powerful telescope to penetrate, all of which may justly be included in the universe, yet it is generally recognized