Page:A Voyage in Space (1913).djvu/62

42 the tail by comparing two photographs taken one after the other. Here is a pair taken by that skilful photographer, Professor Barnard, of the Yerkes Observatory in America. He was the first man to show these beautiful effects. Early in the night he got one picture of the tail; then, by waiting an hour or two, he got another, showing how the tail had changed in the interval. Finally he has very skilfully combined the two pictures, so that the stars on one fall exactly on the stars in the other. You see how the head of the comet has moved, and that the tail is certainly not following it like smoke following an engine. To trace the changes in the