SCIENCE. 



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cases for such observatories to assume the positions of the I 

 fundamental stars, and to leave the fnrther improvement of i 

 their places to skillful astronomers who understand the ! 

 theory of such work, and who carefully study and become j 

 masters of their instruments. In these refined observations 

 the refraction of light by our atmosphere also plays an im- I 

 portant part, and this question will need to be examined at 

 every observatory that undertakes to do independent work. 

 It is true that every new and good meridian instrument 

 may, and perhaps ought, to contribute something towards j 

 removing constant errors, and giving us a more accurate ] 

 knowledge of a star's position ; but when this position is 

 very well known, the only way for further improvement is 

 through complete and careful observations, and their 

 thorough reduction and discussion. 



In the observations of double stars but little had been 

 done before the present century, and the labors of W, 

 Struve form the real starting-point in this branch of astron- 

 omy. These labors have been ably continued by his son, 

 the present director of the Pulkowa Observatory, and the 

 observations of these two astronomers, extending over a 

 period of nearly sixty years, are of the greatest value for 

 our knowledge of the motions of the double stars. This is 

 a branch of the science into which irregular workers are apt 

 to enter, and where some of them have done good service ; 

 but it any amateur astronomer will compare his own work 

 with that of the Struves, and will study the methods fol- 

 lowed by them in determining their personal and instru- 

 mental errors, and will emulate the steadiness with which 

 they have followed out their purpose, he can do much to 

 enhance the value of his labor. Here the observations are 

 simple, and easily reduced, and the chief requisites are skill 

 and patience on the part of the observer. He should not 

 be discouraged because he obtains no immediate or great 

 reward for his work, or public notice, or because some one 

 who rants about the nebular hypothesis and kindred sub- 

 jects, of which he knows nothing, is for a time the great as- 

 tronomer of the day. The observer will learn finally that 

 a good observation of the smallest double star, or of the 

 faintest comet or asteroid, is worth more than all such 

 vague talk. The observation has a positive value, how- 

 ever small, but the physical theories of the universe, of 

 which modern popular science is so productive, are gen- 

 erally worse than useless. 



The first step towards a rational and trustworthy knowl- 

 edge of our sidereal universe must come from a determi- 

 nation of the distances of the stars. The solution of this 

 problem was attempted soon after the Copernican theory of 

 our solar system was established, when it was seen that we 

 have a long base line for our measures, or the diameter of 

 the earth's orbit, and it was supposed that the solution 

 would be easy. These early trials were all failures, but 

 they led to some very interesting and important discov- 

 eries, such as Bradley's discovery of the aberration of 

 light ; to the knowledge of the fact that the determination 

 of the parallaxes, or the distances of the stars, although 

 simple in theory, is practically a difficult question ; and 

 then to an improvement in the instrumental means of ob- 

 servation, to a careful study of the methods of observation 

 and the instruments, and to a recognition of the necessity 

 of a complete and rigorous reduction of the observations. 

 An examination of these early attempts is an instructive 

 study. It is only about forty years ago that the solution of 

 this problem was at last attained, and then only by the ap- 

 plication of the most powerful instruments and the best ob- 

 serving skill. An interesting result of the determinations 

 of stellar parallax is obtained at once in the check it puts 

 on speculations concerning the structure of the sidereal 

 universe. The first astronomers who considered the par- 

 allaxes of the stars very naturally assumed that the bright 

 stars are nearer to us than the faint ones, and therefore 

 they observed the bright stars for parallax. Now, while 

 this assumption may be true as a general statement, the 

 actual determinations of parallax show that some of the 

 faint stars which are not visible to the naked eye are much 

 nearer to us than the brightest stars of our northern sky. 

 Again it was assumed that a large proper motion is a cer- 

 tain index of a star's nearness to us ; but observation shows 

 that this also may be an erroneous assumption. This is a 

 problem whose solution is only just begun, but already we 



know enough of its difficulties to see that we need the most 

 powerful micrometrical apparatus that can be brought into 

 use. The invention of some micrometer that, while as ac- 

 curate as the present filar micrometer, would give the ob- 

 server a much greater range of observation, and enable 

 him to select suitable stars of comparison, is something 

 much to be desired. At present the heliometer seems to 

 be the best instrument for observations of this kind. 

 Formerly it was thought that photography would furnish a 

 good method for such delicate determinations ; but so far 

 the photographic methods have not given the necessary de- 

 gree of accuracy in the measurements, and the astronom- 

 ical use of photography is confined mostly to descriptive 

 astronomy, where, especially in solar eclipses, it has ren- 

 dered excellent service. Closely connected with the par- 

 allaxes of the stars and their proper motions is the interest- 

 ing question of determining their motions to or from our 

 sun, according to the theory of Doppler. Here likewise 

 the numerical determinations are so discordant, that we 

 cannot have much confidence in the results. In both these 

 cases we need more powerful apparatus, and a complete 

 and thorough investigation of the methods of observation. 

 Perhaps some of the large instruments now constructing 

 may be employed in these methods, and we may soon have 

 belter results. 



A great advance has been made in catalogueing the fainter 

 stars. This work was begun by the French astronomers 

 nearly a century ago, and was continued by Bessel, Arge- 

 lander, and others. An important step towards the com- 

 pletion of this work was taken by Argelander and his as- 

 sistants in their great catalogue of the approximate posi- 

 tions of 324,198 stars, which was finished in 1861. This 

 census of the stars will soon be extended, we hope, over 

 the whole heavens ; and it already forms the groundwork 

 for the great zone observations of stars now going on in 

 Europe and in this country, and which must be nearly 

 finished. These observations will doubtless reveal many 

 interesting cases of the proper motion of the stars, and will 

 certainly form the basis for a knowledge of the motion of 

 our solar system in space, and for sidereal astronomy gen- 

 erally, such as we have never had before. Our American 

 observatories can render a good service by observing stars 

 of southern declination, since our observatories are ten or 

 twelve degrees farther south than those of Europe, and thus 

 have an advantage of position which ought to be made use 

 of ; and which may serve to unite into a harmonious sys- 

 tem the observations made in the northern and southern 

 hemispheres. The work of mapping the very faint stars 

 near the ecliptic has also been greatly extended, and it is 

 to this extension that we owe the rapid increase in the 

 number of the small planets between Mars and Jupiter. 

 But besides aiding in the discovery of the asteroids, accu- 

 rate charts of the small stars have a permanent value in 

 giving us a knowledge of the heavens at their epoch, and 

 also some idea of the distribution of the stars in space. 



It is an interesting question whether, among the thousands 

 of nebulae that are scattered over the heavens, any of them 

 show changes of form or of brightness. These objects 

 seem to be at least as distant as the stars, and as they have 

 sometimes an area of several degrees, they must be bodies 

 of an enormous extent. That changes are going on in 

 these bodies seems probable, but to be visible at such dis- 

 tances the changes must be very great. In this case there 

 is need of much caution in the discussion of the drawings 

 made at different epochs, and by different astronomers with 

 telescopes of different power ; since the nebulas change 

 their appearance with the telescope used, with different 

 conditions of the air, and with a variation of their altitude 

 above the horizon. Here the excellent photometers that 

 have been recently invented, and which are being so well 

 applied to the determination of the brightness of the stars, 

 may give us assistance. Perhaps also new drawings of the 

 nebulse, and their criticism and discussion, and a full re- 

 cognition of the difficulties of making such drawings, will 

 soon lead to a decision of the question of their change of 

 form. Since the study of the light of the stars with new 

 and improved photometers has now become a specialty, we 

 may look for more exact and continued observations of the 

 variable stars. This is a matter of which we know but 

 : little, and it is one where a persevering observer may do 



