PARALLAX 253 parallactic displacement in declination in practice, how ever, generally throughout the whole year. The differences of declination so observed, after allowing for the effects of refraction, precession, aberration, nutation, and proper motion, afford the means of deducing the parallax of the star. The most notable series of observations of this character are those of Maclear at the Cape of Good Hope, by which he confirmed the results of his predecessor Henderson and those of Peters at Pulkowa in the second section of his work above mentioned. The latter is the most classic work in existence on refined observations of absolute decli nation, and it is by no means certain that, in more modern meridian observations, the work and methods of that dis tinguished observer have been equalled except perhaps at Pulkowa. The minute precautions necessary in such work will be found in Peters s paper above mentioned (see also TRANSIT CIRCLE). But not with all the skill of Peters, nor with every refinement of equipment and obser vation, can the difficulties caused by refraction and minute change of instrumental flexure, &c., be completely over come ; the method of absolute altitudes does not, in fact, respond in accuracy to the demands of the problem. The differential method depends on measuring the difference of declination, of distance, or of position angle between the star whose parallax is to be determined and one or more stars of comparison. It is assumed that the stars most likely to have sensible parallax are those which are remarkable for brilliancy or proper motion, and that the parallaxes of the stars of comparison (having little or no sensible proper motion and faint magnitude) are so small as to be insignificant. So far as our know ledge goes these assumptions are justified. Researches on stellar parallax by these methods have been followed of late years with considerable success. The instruments employed have been the heliometer and the filar micrometer (see MICROMETER, vol. xvi. pp. 243-248), the latter instrument being used in conjunction with an ordinary equatorial (see TELESCOPE). The precautions ECONDJ SCALE OF T ME IN VEARS FROM 1082-0 3F ARC ,-0 5 -0 4 -O 3 -0 2 -0 1 O 40 1 40 - I D
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- ,, . KMM , """ -rs* Fig. 2. required to determine and eliminate systematic error, and to secure the necessary refinement of accuracy, demand more space for their description than the limits of this article admit. The reader is referred for these particulars to the undermentioned papers on the subject. The heliometer method seems to present the greatest facilities for extensive researches on stellar parallax, not only because measures with this instrument seem, on the whole, to possess the highest accuracy, but because (on account of the large angles that can be measured) a much wider selection of suitable stars of comparison is available. Gylden of Stockholm has applied the method of observing the differences of right ascension between the star whose parallax is to be determined and each of two comparison stars, and the same method has also been applied by Auwers (Math. Abhand. Berliner Acad., 1867) ; but the results obtained in this way do not compare at all favourably with the accuracy of properly conducted helio meter measu The diagram (fig. 1) represents observations made by Gill to determine the parallax of a Centauri, with a helio meter at the Cape of Good Hope. The ordinates of the curve are the time reckoned from 1832 0, the abscissae the changes in the place of a Centauri due to the parallax computed from the observations. Each dot represents the observations of each single night, and the reader will be able to judge of the accuracy of the observations from the agreement of the dots with the curve. Fig. 2 in like manner represents a series of observations of Sirius. These and many other results show that, with similar means, it is now possible to detect any differential parallax amounting to 0" 0o with certainty, by a series consisting of a reasonable number of like observations thus opening up a wide and important field for future research. The following table contains a list of those stars of which the parallax is known with considerable accuracy, Nos. 1 to 13 being in the northern and Nos. 14 to 22 in the southern hemisphere. 1 Magnitude. 1. 61 Cygni 6 2. Lalande 21185 . 7k 3. a Tauri 1 4. 5. 34 Groombridge Lalande 21258 8 01 6. 0. Arg. 17415 9 7. tr Draconis 5i 8. a Lyrte 1 9. p Ophiuehi 44 10. a Bootis.. 1 11. 12. Groombridge 1830.... Bradley 3077 7 6 13. 85 Pegasi 6 14. o Centauri 1 15. Shins 1 16. Lacaille 9352 . 7i 17. e Indi 5? 18. o.y Eiidani .. 4i 19. e Kridani 44 20. CTucanse . . 6 21. Canopus 1 22. j8 Centauri 1 Proper Motion. Parallax. 5 l4 o"-50 4-75 0-50 0-19 0-52 2-81 0-29 4-40 0-26 1-27 0-25 T87 0-24 0-31 19 1-0 0-17 2-43 0-13? 7-05! 0-09 2-09 0-07 1-38 0-05 3-67 075 1-24 0-38 6-95 0-28 4-68 0-22 4-10 0-17 3-03 014 2-05 0-06 o-oo Insensible. Insensible. 1 Authorities. 1. 0. Struve, Mem. Acad. < Petersbourg, ser. vii. vol. i. p. 45 (0" 506); Auwers, Ast. iVach., 1411-16 (0" 56); Ball, Dunsink Observations, vol. iii. p. 27 (0" 465); Hall, Wash. Observa tions, 1879, Appendix I. (0" 478 014). 2. Winnecke (helio meter), Pub. Astron. Gesellsc.haft, No. xi. (0" 5010"-011). 3. O. Struve, Men. Notices R. A. S., vol. xliv. p. 237. 4. Auwers (differ ences of R.A.), Math. Abhand. Berliner Acad., 1807 (0" : 292 0""036). 5. Auwers (heliometer), Astron. Nachrichten, No. 1411 (0" 271 011); Krueger (heliometer), Man. Notices R. A. S., vol. xxiii. p. 173 (0" 2600"-020). 6. Krueger (heliometer), Ibid., (0" 247 021). 7. Brunnow, Dunsink Observations, vol. ii. p. 31 (0"-2400"-011). 8. 0. Struve, Mem. Acad. St Peters-
