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SCIENCE 



[N. S. Vol. XXVI. No. 669 



moved considerably since the first reliable 

 measures were made are accompanied by dia- 

 grams, which by picturing the movement give 

 a clearer indication of the nature of the 

 change than would be possible in any other 

 way. As a rule, observations are quoted 

 sparingly, often not in sufficient numbers to 

 enable one to form an independent opinion 

 respecting the motion. The proper motions 

 are usually quoted from the latest reliable 

 sources, and converted into direction and dis- 

 tance so as to be immediately available in this 

 department. With the exception of Beta 

 Delphini, the only element given of the orbits 

 of the binaries is the periodic time. This is 

 the element that seems to interest the author 

 most. The others are also of importance, and 

 indeed necessary if exact comparisons between 

 observed and computed places are to be made, 

 and it is a distinct loss not to have them 

 quoted also. 



The absence of collected information re- 

 specting the double stars has in the past made 

 it very difficult for observers to arrange pro- 

 grams which should have the maximum effect- 

 iveness in promoting the advancement of this 

 department of science. Many popular stars 

 have been measured repeatedly, sometimes 

 quite unnecessarily, while hundreds of others 

 have been neglected for long intervals. This 

 condition is entirely changed by the appear- 

 ance of these volumes, and in consequence 

 advance from now on should be accelerated. 

 Professor Burnham has endeavored to bring 

 the histories of all pairs as nearly to date as 

 possible, and this has necessitated the re- 

 observation of the neglected stars. For sev- 

 eral years he has devoted himself to this task, 

 and among the observations presented in the 

 notes are the mean results of several thousand 

 measures made for this special purpose, as 

 many as most observers would make in a life- 

 time, and forming a large and important con- 

 tribution to knowledge. 



The value of this work as a reference is 

 enhanced by the tables following the introduc- 

 tion. Here, in remarkably compact form, the 

 double stars discovered by modern observers, 

 that is, since about 1840, are conveniently 



indexed, so that any pair of importance, what- 

 ever its designation, may be very readily 

 found. These indexes are followed by a pro- 

 visional grouping of the double stars into 

 classes according to their motions, so far as 

 these may now be determined. Convenient 

 precession tables are also provided, for the 

 reduction of star places from one epoch to 

 another. 



Even with the data available, as given in 

 these volumes, it is not possible to make more 

 than a beginning in the separation of the 

 double stars into their various classes. Ac- 

 cording to the tables given, the number in 

 each class is as follows: 



Binaries with computed orbits 88 



Binaries without computed orbits 94 



Stars probably binary 112 



Stars of the type of 61 Cygni 38 



Stars with common proper motion 579 



Stars with rectilinear motion 387 



Total 1,298 



These lists include less than ten per cent, of 

 the stars which have been catalogued as 

 double, and to a certain extent they serve as 

 an index to the slowness with which changes 

 in the relative positions of the stars take 

 place. The great majority of all the pairs 

 listed have either remained sensibly fixed, or 

 have moved so little since their first measures 

 were made that it is not now possible to 

 classify them in respect to their movements. 

 In the course of time many more will doubt- || 

 less be added to the lists above, and rapid 

 accessions may be expected in a few years 

 when the close double stars which have re- 

 cently been discovered in such large numbers 

 come to be remeasured. To the present the 

 information of value resulting from double- 

 star investigations, rich as it is, has come 

 from a comparatively small number of objects, 

 hardly more than enough to furnish types. 

 In the future, statistical studies, which are 

 now of precarious value, will doubtless hold 

 an important place and yield many interesting 

 results. Professor Burnham has realized this 

 to the full. He has preferred to rely upon 

 the substantial facts derived from observation 

 rather than upon insecure theory and specula- 



