Attempts have been made on the basis of the contraction theory to find out the past history of the sun and to predict its future. Probably 20 to 50 millions of years in the past represents the life of the sun much as it is at present; and if solar radiation in the future is maintained substantially as now, the sun will have shrunk to one-half its present diameter in the next five million years. So far then as heat and light from the sun are concerned, the sun may continue to support life on the earth not to exceed ten million years in the future. But the sun's own existence, independently of the orbs of the system dependent upon it, might continue for indefinite millions of aeons before it would ever become a cold dead globe; indeed, in the present state of science, we cannot be sure that it is destined to reach that condition within calculable time. A few words on observing the sun, an object much neglected by amateurs. On account of the intense light, a very slight degree of optical power is sufficient. Indeed a piece of window glass, smoked in a candle flame with uniform graduation from end to end, will be found worth while in a beginner's daily observation of the sun. The glass should be smoked densely enough at one end so that the sunlight as seen through it will not dazzle the eye on the clearest days. At the other end of the glass, the degree of smoke film should not be quite so dense, so that the sun can be examined on hazy, foggy or partly cloudy days. An occasional naked-eye spot will reward the patient observer. If a small spyglass, opera glass or field glass is at hand, excellent views of the sun may be had by mounting the glass so that it can be held steadily pointed on the sun, and then viewing the disk by projection on a white card or sheet of paper. Care must be taken to get a good focus on the projected image, and then the faculæ, or whitish spots, or mottling nearer the sun's edge will usually be well seen. By moving the card farther away from the eyepiece, a larger disk may be obtained, in effect a higher degree of magnification. But care must be used not to increase it too much. Keep direct sunlight outside the tube from falling on the card where the image is being examined. This is conveniently done by cutting a large hole, the size of the brass cell of the object glass, through a sheet of corrugated strawboard, and slipping this on over the cell. In this way the spots on the sun can be examined with ease and safety to the eye.