TRANSACTIONS OF THE SECTIONS. t) 



material of incalculable value as the basis of a selenographic map of absolute trust- 

 worthiuess, and also for the solution of the great problem of the moon's physical 

 libration. This question can be solved with certainty by a series of systematic 

 measurements of the distance of definite lunar points firom the limb. Mr. EUerj', 

 Director of the Observatory of Melbourne, has sent over an enlargement of a lunar 

 photograph taken with the Great jNIelbourne Telescope, in which the primary 

 image is 8^^ inches in diameter. Such lunar negatives would be admirably adapted 

 for working out the problem of the physical libration, and also for fuudamental 

 measm-ements for a selenographic map ; the more minute details, however, would 

 have to be supplied by eye-observations, as the best photograph fails to depict all 

 that the eye sees with the help of optical appliances. On the other hand, seleno- 

 graphic positions would be artbrded more free from error than those to be obtained 

 by direct micrometrical measurements. 



' Although, as I have st<ated, I do not contemplate passing in review recent dis- 

 coveries in astronomy, I nmst not omit to call your attention to some few 

 subjects of engrossing interest. First, with reference to the more recent work of 

 Dr. Huggins. In his observations he foimd that the brightest line of the three 

 bright lines which constitute the spectrum of the gaseous nebulas was coincident 

 with the brightest of the lines of the spectrum of nitrogen ; but the aperture of his 

 telescope did not permit him to ascertain whether the line in the nebulas was 

 double, as is the ca,se with the line of nitrogen. With the large telescope placed 

 in his hands by the Royal Society, he has found that the line in the nebulas is not 

 double, and in the case'of the great nebula in Orion it coincides in position with 

 the less refi-angible of the two lines which make up the corresponding nitrogen- 

 line. He has not yet been able to find a condition of luminous nitrogen in which 

 the line of this gas is single and narrow and defined like the nebular line. 



He has extended the method of detecting a star's motion in the line of sight by 

 a change of refrangibility in the line of a terrestrial substance existing on the star 

 to about 30 stars besides Sirius. The comparisons have been made %\'ith lines of 

 hydrogen, magnesium, and sodium. In consequence of the extreme difficulty of the 

 investigation, the numerical velocities of the stars have been obtained by estima- 

 tion, and are to be regarded as provisional only. It will be observed that, speaking 

 generally, the stars which tlie spectroscope shows to be moving fi'om the earth, as 

 Sirius, Betelgeux, Rigel, Procyon, are situated in a part of the heavens opposite to 

 Hercules, towards which the sun is advancing ; while the stars in the neighbour- 

 hood of this region, as Arcturus, Vega, and a Cygni, show a motion of approach. 

 There are, however, in the stars already observed, exceptions to this general state- 

 ment; and there are some other considerations, as the relative velocities of the 

 stars, which appear to show that the sun's motion in space is not the onh^ or even 

 in all cases the chief cause of the observed proper motions of the stars. In the ob- 

 served stellar motions we have to do probably with two other independent motions — 

 namely, a movement common to certain groups of stars and also a motion peculiar 

 to each star. Thus the stars /3, y, S, e, ( of the Great Bear, which have similar 

 proper motions, have a common motion of recession ; while the star a of the same 

 constellation, which has a proper motion in the opposite direction, is sho-mi by the 

 spectroscope to be approaching the earth. From further researches in this direc- 

 tion, and from an investigation of the motions of stars in the line of sight in con- 

 junction with their proper motions at right angles to the visual direction obtained 

 by the ordinary methods, we may hope to gain some definite knowledge of the con- 

 stitution of the heavens. 



This discovery supports, in a somewhat striking manner, the views which 

 Mr. Proctor has been urging respecting the distribution of the stars in space. 

 According to these views there exist within the sidereal system subordinate 

 systems of stars forming distinct aggregations, in which many orders of real mag- 

 nitude exist, while around them is relatively barren space. He had infen-ed the 

 existence of such systems from the results of processes of equal-surface charting 

 applied successively to stars of gradually diminishing orders of brightness. H-e 

 found the same regions of aggregation, whether the charts included stars to the 

 sixth order only or were extended, as in his chart of the northern heavens, to the 

 tenth and eleventh orders ; and these regions of aggregation are the very regions 



1872. . 2 



