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Nature volume 30, pages 181182 | Download Citation



RECENT IMPROVEMENTS IN ASTRONOMICAL INSTRUMENTS.—Acting under the directions of the Secretary of the Navy, Prof. Newcomb last year visited the principal Observatories on the continent of Europe for the purpose of collecting information relating to the most recent improvements in astronomical instruments and methods of observation; and in a Report which has been laid before Congress and printed he has embodied the main results of his journey. The establishments visited were the Observatories of Paris, Neuchâtel, Geneva, Vienna, Berlin, Potsdam, Leyden, and Strasburg, and the workshop of Messrs. Repsold at Hamburg. Prof. Newcomb acknowledges the cordial reception he met with from the directors and astronomers of the various observatories, and the facilities everywhere afforded him for the execution of his mission. Most interest attached to the great refractor constructed for the Observatory at Vienna by Howard Grubb of Dublin, which was completed in 1881, but, owing to various delays, had hardly been brought into active operation at the time of Prof. Newcomb's visit in April 1883. Nevertheless he was able to compare it in several respects with the great Washington telescope, which is of only one inch less aperture. He considers that “as a piece of mechanical engineering it reflects great, credit upon its designer and constructor.” The chief drawback he remarked, the reasons for which were not evident either to him or to Dr. Weiss, the Director of the Observatory, consisted in the failure of the friction-rollers for easing the motion in declination; this motion was found much more difficult than in the case of the Washington telescope. Prof. Newcomb also points to the absence of any rough setting either in right ascension or declination, and the impossibility of seeing the pointing in declination except when the observer was at the eyepiece. With regard to the objective he considers, from such observations as he was able to make, that, “if any defects exist, they are so minute as not to interfere in any important degree with the finest performance of the instrument,” and its proper figuring is rightly considered the most difficult task in the construction of a large telescope. In the workshops of Messrs. Repsold at Hamburg Prof. Newcomb had the advantage of meeting M. Otto Struve, and discussing with him the arrangements for mounting the 3O-inch refractor intended for the Imperial Observatory at Pulkowa, the most striking feature in which is the absence of friction-rollers from the declination axis; he describes the system of wheelwork destined to obviate the difficulty of turning so large an instrument either by hand or a rope attached to the two ends of the axis, as at Washington and Vienna, owing to the amount of the friction. The eyepiece micrometers, as now constructed by the Repsolds, are commended for their rapid and convenient use. Amongst his general practical conclusions Prof. Newcomb expresses the opinion that in the mounting of instruments of the larger size, in order to secure necessary stiffness with the least weight, the axes should be hollow. He does not consider that it is worth while to attach friction-rollers to the declination axis, unless further experiment should show that they can be rendered more effective than in the Vienna equatorial. The old system of attaching a single finder to that side of the telescope which is opposite the declination axis, he remarks, is insufficient in the case of a large instrument, owing to the necessity of setting the opening in the dome not only to the telescope but to the finder, and suggests the desirability of adopting the plan in the Vienna instrument, which has two finders, the one above and the other below the telescope when in the meridian—a plan obviating all difficulty. The Report further explains the principle of the equatorial coudé, or elbow-shaped equatorial, of the Paris Observatory. The Strasburg meridian-circle, “commonly considered to embody the latest conceptions in astronomical mechanics,” is noticed in some detail; Prof. Newcomb thinks a degree of stability has been secured in it which has never before been reached, and he was at much pains to obtain data for comparing the instrument with the meridian-circle at Washington; its general design he describes as similar to that of the great meridian-circle at Harvard College Observatory, which was constructed by Troughton and Simms of London. The reader must be referred to the Report for other particulars bearing upon meridian instruments.

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