But the facts to-day are very different. The working astronomer is an active man in the prime of life, often a young man. He wastes no time in star-gazing. His observations consist of exact measurements made in a precise, systematic, and almost business-like manner. A night's "watch" at the telescope is seldom allowed to exceed about three hours, since it is found that more continued exertions fatigue the eye and lead to less accurate results. To this, of course, there have been many notable exceptions, for endurance of sight, like any form of physical strength, differs greatly in different individuals. Astronomical research does not include "picking out" the constellations, and learning the Arabic names of individual stars. These things are not without interest; but they belong to astronomy's ancient history, and are of little value except to afford amusement and instruction to successive generations of amateurs. Among the instruments for carefully planned measurements of precision the heliometer probably takes first rank. It is at once the most exquisitely accurate in its results, and the most fatiguing to the observer, of all the varied apparatus employed by the astronomer. The principle upon which its construction depends is very peculiar, and applies to all telescopes, even ordinary ones for terrestrial purposes. If part of a telescope lens be covered up with the hand, it will still be possible to see through the instrument. The glass lens at the end of the tube farthest from the observer's eye helps to magnify distant objects and make them seem nearer by gathering to a single point, or focus, a greater amount of their light than could be brought together by the far smaller lens in the unaided eye. The telescope might very properly be likened to an enlarged eye, which can see more than we can, simply because it is bigger. If a telescope lens has a surface one hundred times as large as that of the lens in our eye, it will gather and bring to a focus one hundred times as much light from a distant object. Now, if any part of this telescope be covered, the remaining part will, nevertheless, gather and focus light just as though the whole lens were in action; only, there will be less light collected at the focus within the tube. The small lens at the telescope's eye-end is simply a magnifier to help our eye examine the image of any distant object formed at the focus by the large lens at the farther end of the instrument. For of this simple character is the operation of any telescope: the large glass lens at one end collects a distant planet's light, and brings it to a focus near the other end of the tube, where it forms a tiny picture of the planet, which, in turn, is examined with the little magnifier at the eye-end.