'54 



NA TURE 



[June 15, 1882 



merit he worked indefatigably for thirteen years, making 

 above 10,000 measures ; and it may be said that by this 

 telescope the genius of its maker carried the palm on be- 

 half of refractors in measuring minute quantities in the 

 sky, while the reflectors stepped into the background, and 

 were subseqnently preferred only in cases where the 

 definition is of less consequence than light-grasping 

 power. 1 Struve not only made measures- — thanks to 

 Fraunhofer's excellent micrometer and his skill in 

 handling it — more accurate than had been possible up to 

 that time, but he also catalogued about 3000 double stars 

 between the pole and fifteen degrees southern declination. 

 He had their places exactly determined with Ertel's 

 meridian-circle, and these observations, compared with 

 those of later date, have in many cases established the 

 fact that the proper motion was common for two stars, 

 that revolved so slowly that no change in their relative 

 position had been discovered by aid of the micrometer. 

 Thus their physical connection is then established, but 

 indeed "optical double stars " are so uncommon within 

 the limits here considered, that the discovery of an 

 optical couple is almost a greater curiosity. In such a 

 case the micrometric measures serre to accurately fix the 

 amount of the proper motion of one star, the other being 

 generally so distant that it appears stationary, as well as 

 to ascertain the parallax of the nearer star if perceptible. 

 Struve also every night carefully noted the magnitude and 

 colour of the stars he observed, and divided them into 

 Lucida and reliqua, according to whether the smallest 

 star is above or below the eighth magnitude. According 

 to their mutual distance, he divided them into eight 

 classes, as follows : — 



Class I. Distances from o to 1 

 ,. II. ,. 1 to 2 



,1 HI- ,, 2 to 4 



>■ IV ,, 4 to S 



Class V. Distances from 8 to 12 

 ,, VI. „ 12 to 16 



,, VII. „ 16 to 24 



1, VIII. „ 241032 



Struve's principal works are : " Stellarum duplicium et 

 multipliciura mensurae micrometricae per magnum Fraun- 

 hoferi tubum annis a 1824 ad 1837 in Specula Dorpatensi 

 institutae," and "Stellarum fixarum imprimis composi- 

 tarum positiones mediae dednctae ex observatioribus 

 meridianis a 1822 ad 1843 in Specula Dorpatensi 

 institutis.'' 



Though Struve achieved his main results after the 

 arrival of Fraunhofer's refractor, he had made double- 

 star observations as early as 1814, but his apparatus were 

 then so deficient, that he had to try to make use of dif- 

 ferences of right ascension observed with a small transit 

 instrument, an attempt that, in spite of his experience as 

 an observer, could not but prove a failure. His observa- 

 tions were subsequently continued, under his direction, 

 by his son, who, with the I4|-inch refractor at Pulkowa, 

 discovered about 500 additional objects. He has made 

 about 7000 measures during the las; forty years, and thus 

 we are in possession of observations continued during 

 about seventy years by the Struves. after the same 

 methods. 



Meantime, similar investigations had made consider- 

 able progress elsewhere. In England, the subject was 

 taken up by the Rev. W. R. Dawes, who, taking into con- 

 sideration the smallness of his means, achieved more 

 than any contemporary observer. He is justly considered 

 one of the most distinguished of those amateur astrono- 

 mers, to whom British science is so much indebted. He 

 made about 2000 measures in all. Subsequently, Baron 

 Dembowski, in Italy, commenced micrometric observa- 

 tions of double stars, and though the means originally at 



' ,T he d 5 nnili '' n of an im: >e<! seen in a large reflector is inferior lo [hat in a 

 smaller refractor, both ..n account of the greater influence of any defect in 

 gr.nding the surface cf a mirror, and beeau e. when the aperture is larger 

 the rays of light from the object have in pass through JO much larger a p jr- 

 Uon of the atmosphere, the irregularities and motions in which render the 

 mage unsteady and badly 



his disposal would have been wholly inadequate in other 

 hands, the accuracy of his measures was about as great 

 as that attained at more richly-furnished observatories. 

 Pushed on by his success, he acquired larger and better 

 instruments from Fraunhofer's successor at Munich, and 

 entered upon a series of observations, in which he greatly 

 surpassed the accuracy of other observers. It is there- 

 fore to be regretted that the mass of observations he 

 accumulated during a quarter of a century, has not yet 

 been more than partly laid before the public. Investiga- 

 tors were, however, expecting a volume that would com- 

 pletely embody Dembowski' s work, when the mournful 

 news of his death in January, 1881, spread over Europe. 

 Compared to his observations, those made by Sir W. 

 Herschel appear to be as rude as observations made 

 before the invention of the telescope, compared to those 

 of the nineteenth century. 



It was in the course of the researches carried on by the 

 I latter observers, that circumstances came to light which 

 have proved to be of the utmost importance. I allude 

 to the existence of systematic errors. Already Struve 

 found that he measured angles of position differently, 

 when he inclined his head to either side, and he found 

 that in any case, his distances were different from those 

 given by other observers. He did not, however, follow 

 up this remark, but merely kept his head straight while 

 observing, and with regard to the distances he did not 

 see how his own results could deviate from the truth ; but 

 his son, though he is in possession of such a great re- 

 fractor, has been found to measure double stars altogether 

 erroneously. This he has remedied by observing artificial 

 double stars (white ivory disks on a black ground), and 

 after applying the corrections thus ascertained to his 

 measures on the sky, the accuracy of his results has been 

 sensibly increased, though of course the circumstances 

 attendant on such operations are very different from those 

 under which astronomical observations are made during 

 the night, e.g. the artificial double stars are always seen 

 near the horizon and are stationary, while the stars are 

 ever moving, and have to be followed by aid of a more or 

 less deficient clockwork driving the telescope, Dawes 

 also, found systematic errors in his measures. He tried 

 to do away with them by slightly inclining his head when 

 the stars were nearly in a vertical, and by the use of 

 a prism, fixed before the eyepiece, to make them appear 

 vertical, when the line joining their centres formed a great 

 angle with the vertical. He says, in the introduction to 

 his ob=ervations, that no one about to draw a straight 

 line with a ruler would lay this crooked on the table ; one 

 prefers to lay it parallel to the line joining the two eyes. 

 It is in fact most agreeable to measure a double star 

 when the components are either nearly vertical or nearly 

 horizontal. Dembowski' s observations seem free from 

 systematic errors, but with praiseworthy diligence he has 

 thought fit to subject this circumstance to a minute 

 scrutiny. To this end he was observing circumpolar 

 double stars of different classes in every hour angle round 

 the pole, as these errors have been found to depend upon 

 not only the angle the line joining the stars makes with 

 the line joining the eyes of the observer, but also 

 on their mutual distance, and as the error diminishes 

 quickly as this increases, it is recommended to use 

 always the highest magnifying power which the state 

 of the atmosphere and the quality of the object-glass 

 will allow. 



Space would not allow me to refer to all the astro- 

 nomers, who at one time or another have prid attention 

 to the subject, or to discuss the relative value of their 

 work. Father Secchi made some good measures in Italy, 

 Duner, in Sweden, has published about 3000 valuable 

 observations, and Gledhill, in Halifax, has also success- 

 fully taken up this work. In spite of the skies of Con- 

 naught, that clear so seldom and so irregularly, I have 

 tried to do my best, but I have not succeeded in getting 



