106 ON THE MAGNITUDE OF THE SOLAR SYSTEM. 



light corpuscles impinging upon the earth were thought to behave pre- 

 cisely like tlie drops in a shower of rain, and you all know that their 

 apparent direction is affected by any motion on the part of the observer. 

 In a calm day, when the drops are falling perpendicularly, a man stand- 

 ing still holds his umbrella directly over his head, but as soon as he 

 begins to move forward he inclines his umbrella in the same direction, 

 and the more rapidly he moves the greater must be its inclination in 

 order to meet tlie descending shower. Similarly, the apparent direc- 

 tion of oncoming light corpuscles would be affected by the orbital 

 motion of the earth, so that in effect it would always be the resultant 

 arising from combining the motion of the light with a motion equal and 

 opposite to that of the earth. But since the falsity of the corpuscular 

 theory has been proved that explanation is no longer tenable, and as 

 yet we have not been able to replace it with anything equally satis- 

 factory based on the now universally accepted undulatory theory. In 

 accordance with the latter theory we must conceive the eartli as plow- 

 ing 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 phenomena 

 of light, and for two hundred years it was not suspected of having any 

 other function. The emission theory postulated only the corpuscles 

 which constitute light itself, but the undulatory theory tills all space 

 with an imponderable substance jjossessing properties even more 

 remarkable 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 Brews- 

 ter, who had gained a world-wide reputation by his optical researches, 

 expressed himself 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 

 18-t5, when Faraday first performed his celebrated experiment of mag- 

 netizing 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 proi)agation of an electro-magnetic 

 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 electro-magnet unit, and careful experiments soon proved 

 that that is the velocity of light. Thus it was put almost beyond the 

 I^ossibility of doubt that the ether gives rise to the phenomena of elec- 

 tricity and magnetism as well as to those of light, and perhaps it may 



