In the year 128 B. C. Hipparchus put out a catalogue of 1025 stars observed at Rhodes. Twenty such works succeeded this up to the year 1801, when Lalande, of Paris, brought out a list of 47,390 stars. It will be remembered that few stars have names, except those known to the Arabians of old, but are designated by their positions in the heavens. It is customary to refer to them by their declinations and right ascensions, as so many degrees north or south of the celestial equator, and so many degrees, or hours, east of the vernal equinox—fifteen degrees being the equivalent of an hour of right ascension—just like the latitude and longitude of cities on a common globe. During the nineteenth century many celestial atlases and astronomical catalogues have been published. These contain lists of comets and nebulæ, and the places of the double stars and of the fixed stars. Of the latter alone over one hundred have appeared, of which Argelander’s is by far the largest, as it contains the places of more than 310,000 stars. The catalogue prepared by the British Association in 1845 is of great value, containing 8377 stars. Yarnall’s, of 10,658 stars, published in Washington in 1873, is most accessible to us. Professor C. H. F. Peters, of the Hamilton College Observatory, Clinton, N. Y., the discoverer of so many asteroids, has prepared a valuable series of star charts. By dividing the heavens into small squares and carefully photographing each of them, the places of a vast number of stars can be recorded with far greater accuracy than by the old plan of a separate instrumental measurement of the position of the stars. By the use of microscopes the determination of their positions can be made with precision. These plates are preserved with care, and when those of the same region of the skies, made in different years, are compared, any variation in the relative positions of the objects can be detected with certainty. The perfection of this method of star-mapping is justly deemed one of the most important achievements of the century. For an amateur star-gazer who is not provided with a set of maps, Whitall’s Planisphere is a very ready aid, as it can be instantly adjusted to any day and hour. The inexperienced, and those who have no instruments, can use it with ease and satisfaction to locate a thousand of the most conspicuous stars. XII. ASTRONOMICAL BOOKS AND THEIR WRITERS. In England this attractive study has been popularized chiefly by the interesting works of the two Herschels, who were voluminous writers, the lectures of Proctor, and the admirable compend of facts so assiduously gathered by G. F. Chambers in his delightful treatise on astronomy.