572 



NA TURE 



{Oct. 14, 1 8 So' 



on account of this fact that Bessel's tables, published in 1S30, 

 were of such great service, since they introduced correct and 

 elegant methods of reduction, and clearly defined all the constants 

 and epochs. We now have the positions of several hundred 

 stars so well known that they may be safely used in the reduction 

 of ob^ervations ; and for these accurate positions we are largely 

 indebted to the astronomers of the Pulkowa Observatory, who 

 have made such absolute determinations a special work, Tliere 

 is still an opportunity for the improvement of these positions, 

 and every well-executed determination will be of value ; but it is 

 doubtful if crude and irregular observations can add anything to 

 our knowledge of the positions of these stars. Neither can the 

 routine, mechanical style of observing, that is apt to prevail in 

 large observatories, be of much use here. It would be better in 

 most cases for such observatories to assume the positions of the 

 fundamental stars, and to leave the further improvement of their 

 places to skilful astronomers who under.-tand the theory of such 

 work, and who carefully study and become masters of their 

 instruments. In these refined observations the refraction of 

 light by our atmosphere also plays an important part, and tliis 

 question will need to be examined at every observatory tliat 

 undertakes to do independent work. It is true that every new 

 and good meridian instrument may, and perhaps ought, to cm- 

 tribute something towards removing constant errors, and giving 

 us a more accurate knowledge of a star's position ; but wlien 

 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 labours of W. Struve form 

 the real starting-point in this branch of astronomy. These labours 

 have been ably continued by his son, the present director of the 

 Pulkowa Observatory, and the observations of these two astro- 

 nomers, 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 if any amateur astronomer will compare his oun Avorlc with 

 that of the .Struves, and will study the methods followed Ijy 

 them in determining their personal and instrumental errors, and 

 will emulate the steadiness with which they have followed out 

 their purpose, he can do much to enhance the value of his labour. 

 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 sul:]jects 

 of which he knows nothing is for a time the great astronomer 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 ob^ervation hai a 

 positive value, however small, but the physical theories of the 

 universe, of which modern popular science is so productive, are 

 generally worse than useless. 



The first step towards a rational and trustworthy knowledge 

 of our sidereal universe must come from a determination of tire 

 distances of the stars. The solution of this problem was at- 

 tempted 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 im- 

 portant discoveries, 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 im- 

 provement in the instrumental means of observation, to a careful 

 study of the methods of observation and the instruments, and to 

 a_ recognition of the necessity of a complete and rigorous reduc- 

 tion of the observations. An examination of these early attempts 

 is an instructive study. It is only about forty years ago that the 

 solution of thii problem was at last attained, and then only by 

 the application of the most powerful instruments, and the best 

 observing skill. An interesting result of the determinations of 

 stellar parallax is obtained at once in the chei-k it puts on specu- 

 lations concerning the structure of tlie .sidereal universe. The 

 first astronomers who considered the parallaxes of the stars very 

 naturally assumed that the bright stars are nearer to us than tli'e 

 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 certain 

 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 iMwerfvd micrometrical 

 apparatus that can be brought into use. The invention of some 

 micrometer that, while as accurate as the present filar microme- 

 ter, would give the observer a much greater range of observa- 

 tion, 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 degree of accuracy in the 

 measurements, and the astronomical use of photography is con- 

 fined mostly to descriptive astronomy, wdiere, especially in solar 

 eclipses, it has rendered excellent service. Closely connected 

 \vith the parallaxes of the stars and their proper motions is the 

 interesting 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 investi- 

 gation of the methods of observation. Perhaps some of the 

 large instruments now constructing may be employed in these 

 methods, and we may soon have better results. 



A great advance has been made in cataloguing the fainter 

 stars. This work was begun by the French astronomers nearly 

 a century ago, and was continued by Lessel, Argelander, and 

 others. An iaiporlant step towards the completion of this work 

 was taken by Argelander and his assistants in their great cata- 

 logue of the approximate positions of 324,198 stars, which was 

 finished in 1S61. 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 obsei-vations of stars now going 

 on in Europe and in this country, and which must be nearly 

 fini-hed. Th^se observations will doubtle-s reveal many inter- 

 esting 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 generally, such as we have 

 never h.ad 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 system the obervatious 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 

 tills extension tliat 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, accurate charts of the small 

 stars have a permanent value in giving us a knowdedge 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 

 nebula; that are scattered over the heavens, any of them show 

 changes if 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 distances 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 nebula; change 

 their appearance with the telescope used, with different condi- 

 tions 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 nebulae, 

 and their criticism and discus-ion, and a full recognition of the 

 difficulties (»f 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 continueil 

 observations of the variable stars. This is a matter of which we 

 know but little, and it is one where a persevering observer m>ay 

 do good service. Although he may not find any immediate 

 encouragement in the discovery of remarkable relations among 



