392 THE WORLD MACHINE 



in their apparent brightness by hundreds and perhaps thou- 

 sands of times. Such is the case, for example, of Sirius and 

 61 Cygni. This difference may be due to a vast disparity in 

 size ; it may be due to an equally wide disparity in their tem- 

 perature or light-giving power. The apparent brightness, there- 

 fore, is only the vaguest sort of a clue to the distance. It is 

 actual measures of parallax alone which could ever afford a 

 sure basis for safe induction. 



Proctor likewise brought forward the argument from the 

 apparent drift of the stars. Here and there over the heavens 

 the stars of certain regions, at least such as disclose an apparent 

 motion, seem moving in a common way. Their motions may 

 not be at all equal, but have the same general direction. It 

 is just as if throughout the spaces of the sky there were cur- 

 rents and eddies as in a stream of water. This Proctor aptly 

 described as " star drift." It may be entirely an illusion, a 

 happen-chance, depending on our point of view ; but if it have 

 any underlying reality, it is evident that this would result in 

 agglomerations of stars in one region with relative emptiness 

 in another. If such systems exist, it is clearly hopeless to 

 suppose that our present means are sufficient to attain any 

 definite ideas as to universal structure. 



Carrying out his conception of star drift, Proctor was led 

 to conceive the Milky Way as an irregular spiral stream of 

 minute stars lying in and among the larger stars of the system. 

 That all this could be little more than guess-work is evident 

 from the conclusions of Madler. Drawing naturally upon much 

 the same material, Madler conceived the stars of the Milky 

 Way as entirely separated from the rest of the stellar system 

 and as belonging to an outlying ring or system of rings. In 

 order to account for the gaps in the Milky Way, this ring was 

 supposed to be cleft on one side. It is on this account often 

 referred to as the " cloven ring " theory. In this view the 

 stellar system, viewed from without, might present some such 

 an appearance as a split key-ring, spread out a bit. The outer 

 ring, cleft on one of its sides, would represent the system of the 

 Milky Way, while the luminous mass in the centre would include 

 the remainder of our stellar universe. Madler likewise enter- 

 tained the idea of a central sun. 



It cannot be said that we are any further advanced at the 



