W. Harkness — Magnitude of the Solar System. 245 



and as yet we have not been able to replace it with anything 

 equally satisfactory based on the now universally accepted 

 undulatory theory. In accordance with the latter theory we 

 must conceive the earth as plowing its way through the ether, 

 and the point which has hitherto baffled us is whether or not 

 in so doing it produces any disturbance of the ether which 

 affects the aberration. In our present ignorance on that point 

 we can only say that the aberration constant is certainly very 

 nearly equal to the ratio of the earth's orbital velocity to the 

 velocity of light, but we can not affirm that it is rigorously so. 



The luminiferous ether was invented to account for the phe- 

 nomena of light, and for two hundred years it was not sus- 

 pected to have any other function. The emission theory 

 postulated only the corpuscles which constitute light itself, 

 but the undulatory theory fills all space with an impon- 

 derable substance possessing properties even more remark- 

 able than those of ordinary matter, and to some of the 

 acutest intellects the magnitude of this idea has proved 

 an almost insuperable objection against the whole theory. 

 So late as 1862 Sir David Brewster, who had gained a world- 

 wide reputation by his optical researches, expressed him- 

 self as staggered by the notion of filling all space with some 

 substance merely to enable a little twinkling star to send its 

 light to us ; but not long after Clerk Maxwell removed that 

 difficulty by a discovery coextensive with the undulatory theory 

 itself. Since 1845, when Faraday first performed his celebra- 

 ted experiment of magnetising a ray of light, the idea that 

 electricity is a phenomenon of the ether had been steadily 

 growing, until at last Maxwell perceived that if such were the 

 fact the rate of propagation of an electromagnetic wave must 

 be the same as the velocity of light. At that time no one knew 

 how to generate such waves, but Maxwell's theory showed him 

 that their velocity must be equal to the number of electric 

 units of quantity in the electromagnetic unit, and careful 

 experiments soon proved that that is the velocity of light. 

 Thus it was put almost beyond the possibility of doubt that 

 the ether gives rise to the phenomena of electricity and mag- 

 netism as well as to those of light, and perhaps it may even be 

 concerned in the production of gravitation itself. What could 

 be apparently more remote than these electric quantities and 

 the solar parallax ? And yet we have here a relation between 

 them, but we make no use of it because as yet the same rela- 

 tion can be far more accurately determined from experiments 

 upon the velocity of light. 



Now let us recall the quantities and methods of observation 

 which we have found to be involved either directly or indi- 

 rectly with the solar parallax. They are, the solar parallax, 



