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RESEARCHES IN STELLAR PARALLAX.
The observatory of Trinity college, Dublin, has long been the moat famous spot on earth for determinations of stellar parallax: and the labors of the present astronomer royal of Ireland, Dr. Ball, conducted in the same line of research, will make good the claim of this institution to such distinction for a long time to come. Before the time of Dr. Brümnow, formerly astronomer royal at Dunsink, no astronomers had, except in isolated instances, attacked the problem of stellar parallaxes for its own sake; that is to say, the determinations of parallax had come about rather incidentally, and had not been undertaken with the idea of determining stellar distances as the sole end of the research. The painstaking care which Dr. Brümnow exercised as an observer, and his conscientious thoroughness in the subsequent numerical work based on his observational data, were so skilfully combined as to show that the distances of the stars were readily determinable with a hopeful, and to a certain extent satisfactory, precision. The stars with which he was largely occupied were α Lyrae, Groombridge 1830, 85 Pegasi, and δ Draconis.
Dr. Hull, appointed astronomer royal some ten years ago, has wisely devoted the resources of the Trinity college observatory in the main to parallax research, and he has greatly amplified the plans of his predecessor. In the present volume he details the method by which his observations in systematic search for stars with a large parallax have been conducted; and it is plainly apparent how an enthusiastic worker can completely observe so many stars when special pre-arrangements are adopted for the economy of time and labor. With slight changes, these same methods would be equally applicable to the details of other observatory work, and would result in an equal saving: the methods are quite similar to those now so common in the details of library management, and have already been adopted by many astronomers in facilitating their work.
It will be a matter of interest to many to know how, from the myriads of stars in the sky. Dr. Ball was guided in the selection of a list embracing about a thousand objects. In the first place, only such objects were taken as
were included between 30" ami da" of north declination, and every object of importance in Admiral Smyth's celebrated cycle of celestial objects was transcribed into the working-lists. Struve's catalogue was also drawn upon, and likewise catalogues of red stars by Schjellerup and Birmingham ; the hj-pothesis with regard to objects of this type being that their color may be due to their small size, and thus pre- sumably less far removed from the solar sys- tem. A number of the variable stars, also, are iirobably very small, and tiiey were included in Dr. Bull's lists for a like reason.
It will not be understood that Dr. Ball's work amounts to a conclusive determination of the distances of all these objects ; the ob- jects of his research are at present lery differ- ent from this: and his labors were directed with main reference to a decision, in all cases, whether the observed stars indicate a sufli- ciently large parallactic displacement to merit further immediate attention. Of course, there was no disappointment in finding that a very small proportion of the objects examineii gave satisfactory evidence of a measurable parallax ; but the labors of Dr. Ball are none the less important to future observers as indicating clearly the direction in which there is no pressing need of similar investigation. So much for the inconclusive part of this work. And we may now speak of the positive results in the shape of accurate determinations of the parallax of lil {B) Cvgni, Groombridge KilH, andliCygni (B).
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��The first star belongs to the famous binarj' syst«m, the first determination of whose dis- tance was made by the illustrious Bessel ; and Dr. Ball finds its annual parallax to be very little short of half a second of arc. In order to show the degree of accuracy attained in
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