Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/892

820 and rate of motion as accounts for the greatest proportion of the observed stellar motions. The plan was carried out according to two different hypotheses respecting errors of observation, and was first applied to about 300 stars, afterwards to about 1500, by Mr Dunkin. The results of the first pair of solutions are indicated by the points A z and A a in fig. 52, the results of the others by the point A 3 and A 4. A, lies in 11. A. 261 14 and N.PD. 57 51 ; A 4 in R.A. 263 44 and N.P.D. 65. Lastly, Mr Galloway, by considering the motion of southern stars, has deduced the result indicated at G. Giving to these various results their due weight, we find for the probable position of the apex that shown in fig. 52. —Various Determinations of the Apex of the Sun's Way.

The analysis of the proper motions of the stars by Mr Proctor has led to the recognition of local star-drifts. This is illustrated in fi?. 53. Here arrows are drawn from the several stars, indicating their direction and rate of motion, the rate being such that 36,000 years would be required to carry any star from its present position to the position marked by the extremity of the arrow-point. It will be seen that the stars (3, y, 8, e, and Ursce Majoris, with the companions of, are travelling in the same direc tion and at the same apparent rate. Similar drifting motions can be recognised in other parts of the heavens. —Illustrating Star-drift.

Spectroscopic analysis affords a means of determining the motions of stars in the direction of the line of sight. For if a star is receding, the wave length of any definite portion of its light must be apparently lengthened ; while if the star is approaching, such wave-length must be shortened. Thus, if the position of some known line in a star s spectrum can be compared with the position of the same line in the spectrum of a terrestrial element, any shift of position indicates whether the star is receding or ap proaching. In this way several of the brighter stars have been dealt with by Mr Huggins, and motions of recession and approach, amounting in some cases to 30 or 40 miles per second, have been recognised. This method has afforded an interesting confirmation of Mr Proctor s views about star-drift, seeing that Mr Huggins assigns to the five stars /?, y, 8, e, and Ursas Majoris, equal motions of recession (about 17 miles per second). He also finds that their spectra are alike, indicating a resemblance in constitution and structure, such as we should expect to find in the members of a drifting system.

The Nebulæ are celestial objects which present a cloudy appearance. &quot;When examined with a telescope of moderate power, the greater part of the nebulae are distinctly per ceived to be composed of clusters of little stars, imperceptible to the naked eye, because, on account of their apparent proximity, the rays of light proceeding from each are blended together, through the effects of irradiation, in such a manner as to produce only a confiiscd luminous appearance. In others, however, no individual stars can be perceived, even through the best telescopes, and the nebula exhibits only the appearance of a self-luminous or phosphorescent patch of matter in a highly dilated or gaseous state. In some instances the nebula presents the appearance of a faint luminous atmosphere, of a circular form and of large extent, surrounding a star of considerable brilliancy. One of the most remarkable nebula is that which is situated in the sword-handle of Orion. It was discovered by Huyghens in 1656, and described and figured by him in his Sy sterna Saturninm. Since that time it has been examined and described by various observers, parti cularly Fouchy, Mairan, Le Gentil, and Messier, who have given engravings of it ; and if any trust can be placed in their descriptions of so indistinct and difficult an object, it must have undergone great changes in its form and physical appearance. Unfortunately, however, no satisfactory in ference can be drawn from the comparison of the different descriptions ; for it is found that the same nebula, viewed on the same night with different telescopes, presents appearances so different as to be scarcely recognisable as the same object. The effects of atmospherical variations also cause great differences in its appearance, even when it is viewed through the same telescope at different times, so that it is scarcely possible that any two observers will be found to agree in their delineation of its outline. Sir J. Herschel, in the second volume of the Memoirs of the Astronomical Society, has given a detailed description of this nebula as it appeared in his twenty-feet reflector in 1824, together with a drawing, which, on account of the superiority of his telescope, is probably a much more cor rect representation of the object than any which previously existed. Plate XXX. is copied from that drawing. Of that portion of the nebula which he calls the Huyghenian region Sir J. Herschel gives the following account : &quot; I know not how to describe it better than by comparing it to a curdling liquid, or a surface strewed over with flocks of wool, or to the breaking up of a &quot;mackerel&quot; sky when the clouds of which it consists begin to assume a cirrous appearance. It is not very unlike the mottling of the sun s disk, only (if 