Page:A Voyage in Space (1913).djvu/237

Rh this we need only a screen with a slit in it at the right place; all other colours will be stopped by the screen, but the special red will shine through the slit.

This special colour need not, of course, always be red; we can alter the place of the slit so as to use any other colour we please, which is lucky, because red is not at all a good colour for photography. You probably know that you can have red light in the dark room without spoiling your negative. And, further, we need not only photograph the red flames at the Sun's edge, we can use