"I propose to discuss this afternoon certain effects of the energy which is continuously pouring out from the sun on all sides with the speed of light—the energy which we call sunlight when we enjoy the brilliance of a cloudless sky, which we call heat when we bask in its warmth, the stream of radiation which supports all life on our globe and is the source of all our energy. ... ... Beyond the violet are the still shorter waves, which affect a photographic plate or a fluorescent screen, and will pass through certain substances opaque to ordinary light. Here, for instance, is a filter, devised by Professor Wood, which stops visible rays, but allows the shorter invisible waves to pass and excite the fluorescence of a platinocyanide screen. Again, beyond the red end are still longer waves, which are present in very considerable amount, and can be rendered evident by their heating effect. We can easily filter out the visible rays and still leave these long waves in the beam by passing it through a thin sheet of vulcanite. A piece of phosphorus placed at the focus of these invisible rays is at once fired, or a thermometer quickly rises in temperature."