"Although Halley's comet has come within the earth's orbit about three thousand times since its first recorded appearance, I know of no man living who can give a satisfactory account of having seen it. Any one who has seen it before must be at least seventy-five years old, for it requires seventy-five years to make one complete circuit of its own orbit. But no one who is now seventy-five could have observed it intelligently, and even one who is now eighty-five years old would have to tell what he saw when he was ten years old and has remembered for seventy-five years. Furthermore, any account of how it looked on a former return is no guide to how it may appear on this trip. You may properly think of the comet as a group of solid pieces no bigger than the stones you may throw, scattered, two or three to the mile, through a space 12,500 miles broad. This extremely thin cloud of particles does not reflect enough sunlight to be visible, even in a telescope, in any part of its journey, and hence we should be wholly unaware of its existence if it did not sometimes have the strange faculty of giving out light of its own _while in that part of its own orbit nearest to the sun_. At such a time there is a hazy light enveloping the mass of small bodies, and streaming away sometimes many million miles from them. The mass of small bodies is generally referred to as the nucleus, and the stream of luminous gas which the nucleus gives forth is called the tail, though it reminds me more of a search-light. "It does not trail along behind the comet but always points away from the sun (Fig. 197). The normal thing for a comet to do is to begin to develop a faint light and a short streamer as it gets near to the sun, to have its light grow brighter and its streamer to grow longer until it reaches the point nearest the sun, and then to have its light grow dimmer and the streamer grow shorter as it recedes from the sun. [Illustration: Fig. 197] "It has many times been suggested that this strange search-light appearance may be an electrical phenomenon, some form of ether waves which the comet sends forth when under the immediate influence of the sun. But not all comets are alike in this matter, nor does the same comet always act alike on succeeding trips, so that we may not predict what Halley's comet will do on this visit. It would be natural to suppose that Halley's comet, like radium, might in time lose the power to radiate off material, in which case it might at length become wholly invisible to us, even though it continued to travel in its wonted path. Our only way of knowing of its existence then would be that on its returns some of its small pieces might be attracted to the earth and enter our atmosphere as meteors. This sort of thing is continually happening, and may be the last reminders of once brilliant comets.