THE BIRTH AND DEATH OF WORLDS 419 



In their main features, after half a century, von Helmholtz's 

 conclusions remain unshaken. They do not exclude the con- 

 tribution of the meteorites. If radio-active bodies exist in the 

 sun, they too may share in the sun's support. But the main 

 source of its vast wealth of heat and light is the contraction of 

 its own mass. The power to which Kant and Laplace had 

 attributed the origin of the solar system has likewise dowered 

 it with warmth, with light, with life. 



But the matter of especial interest here is the support which 

 the theories of solar energy have given to the Nebular Hypo- 

 thesis. Newcomb has pointed out that Kant and Laplace 

 reached their conceptions in some sense by reasoning forwards 

 that is to say, they assumed that a nebulous mass once 

 existed. There was no proof of this. They reasoned by a 

 process of deduction. The path followed by Helmholtz was 

 the reverse he .reasoned backwards. His results were attained 

 by an induction from observed facts. He showed that, work- 

 ing back from present conditions, we should inevitably reach 

 precisely such a condition as his forerunners had assumed. 

 That two opposite modes of reasoning from utterly different 

 sets of facts should have thus led to an identical result, gave 

 to the hypothesis a solidity of foundation from which it can 

 in no probability be shaken. Whatever may be the details of 

 planetary evolution, the main fact was now clear. 



Something like finality and, with it, some highly suggestive 

 indications were brought by the camera. When the eager, 

 searching eyes of Herschel had closed their long scrutiny, the 

 number of known nebulae had reached 2500. With the aid of 

 the camera, the number estimated as presently known is above 

 120,000. Utilising the great Crossley reflector at the Lick 

 Observatory, the late Professor Keeler, whose researches in 

 this field disclosed so much that was new, was able to calcu- 

 late that the total number which the camera might eventually 

 record would reach several hundred thousand. In a word, the 

 universe is teeming with them, and their extent is beyond con- 

 ception. They are for the most part so distant that they have 

 no observable parallax. Yet some of them range across several 

 minutes of arc in the telescopic field. It follows, therefore, 

 that some of the nebulae must be thousands of times in extent 

 the diameter of the earth's orbit. Probably a great number 



