than any other event recorded in the annals of the science. Numerous atlases and catalogues of stars have been made within the last half century, but until recently, none which could be regarded as faithful and indubitable representations of the heavens at the date of their construction. Celestial charts were formerly made by merely plotting down observed stars from known catalogues, without subsequent telescopic revision. As may be readily imagined, maps so formed abounded with omissions and errors: stars being frequently inserted of a fainter class than was intended for the limit of magnitude, while numerous brighter ones, were left out altogether. The great catalogues, and most especially the zone observations made under the direction of the eminent astronomers Lalande, Bessel, and Argelander, comprising above one hundred thousand different stars, valuable and accurate as the two latter series especially are, give no definite idea of the stars actually visible with a telescope only two inches in aperture. A survey, worthy of being so called, must be so scrupulously revised, that no single star exceeding its stated limit in brilliancy shall be omitted; and the fact of an object not being inserted therein, must be proof of its inferiority to such photometric limit at the time the survey was made. Unless such accuracy can be ensured, the whole is comparatively of little worth; but with patience and perseverance the desired result, though confessedly very laborious, is nevertheless attainable.
Considerations of this nature induced the distinguished director of the Royal Observatory at Bonn, to survey and map the entire northern hemisphere, including all stars down to the 9½ magnitude, and aided by his well-trained assistants, the enormous task was completed in less than fight years. Professor Argelander and his staff commenced operations in the spring of 1852 and concluded the requisite
