CHAPTERS ON THE STARS. 657 



farther away that it may be regarded as having no parallax. In this 

 assumption lies the weak point of the method. Can we be sure that 

 the smaller stars are really without appreciable parallax? Until recent 

 times it was generally supposed that the magnitude of the stars afforded 

 the best index to their relative distances. If the stars were of the same 

 intrinsic brilliancy, the amount of light received from them would, as 

 already pointed out, have been inversely as the square of the distance. 

 Although there was no reason to suppose that any such equality really 

 existed, it would still remain true that, in the general average, the 

 brighter stars must be nearer to us than the fainter ones. But when the 

 proper motions of stars came to be investigated, it was found that the 

 amount of this motion afforded a better index to the distance than 

 the magnitude did. 



The diversity of actual or linear motion is not so wide as that of 

 absolute brilliancy. Stars have, therefore, in recent times, been selected 

 for parallax very largely on account of their proper motion, -without 

 respect to their brightness. It is now considered quite safe to assume 

 that the small stars without proper motion are so far away that their 

 parallax is insensible. 



Ever since the time of Bessel the experience of practical astronomer?; 

 has tended toward the conclusion that the best instrument for delicate 

 measurements like these is the heliometer. This is an equatorial tele- 

 scope of which the object glass is divided along a diameter into two 

 semicircles, which can slide along each other. Each half of the object 

 glass forms a separate image of any star at which the telescope may be 

 pointed. By sliding the two halves along each other, the images can 

 be brought together or separated to any extent. If there are two stars 

 in proximity, the image of one star made by one-half of the glass can 

 be brought into coincidence with that of the other star made by the 

 other half. The sliding of the two halves to bring about this coinci- 

 dence affords a scale of measurement for the angular distance of the 

 two stars. 



The most noteworthy forward steps in improving the heliometer are 

 due to the celebrated instrument-makers of Hamburg, the Messrs. 

 Repsold, aided by the suggestions of Dr. David Gill, astronomer at the 

 Cape of Good Hope. The latter, in connection with his coadjutor, 

 Elkin, made an equally important step in the art of managing the 

 instrument and hence in determining the parallax of stars. The best 

 results yet attained are those of these two observers and of Peter, of 

 Germany. 



Yet more recently, Kapteyn, of Holland, has applied what has 

 seemed to be the unpromising method of differences of right ascension 

 observed with a meridian circle. This method has also been applied 

 by Flint, at Madison, Wis. Through the skill of these observers, as 



VOL. LVII.— 42 



