ON THE MAGNITUDE OF THE SOLAR SYSTEM. 105 



as a star in the center of the flekl of view. That adjustment being 

 made, if the mirror were caused to revolve at a speed of some hundred 

 turns per second it would move through an appreciable angle while the 

 light was passing from it to the distant mirror and back again, and in 

 accordance with the laws of reflection, the star in the field of the tele- 

 scope would move from the center by twice the angle through which 

 the mirror had turned. Thus the deviation of the star from the center 

 of the field would measure the angle through which the mirror turned 

 during the time occupied by light in passing twice over the interval 

 between the fixed and revolving mirrors, and from the magnitude of 

 that angle, together with the known speed of the mirror, the velocity 

 of the light could be calculated. 



In applying either of these methods the resulting velocity is that of 

 light when traversing the earth's atmosi)here, but what we want is its 

 velocity in space, which we suppose to be destitute of ponderable 

 material, and in order to obtain that the velocity in the atmosphere 

 must be multiplied by the refractive index of air. The correct velocity 

 so obtained can then be used to find the solar parallax, either from the 

 time required by light to traverse the semidiameter of the earth's orbit, 

 or from the ratio of the velocity of light to the orbital velocity of the 

 earth. 



Any periodic correction which occurs in computing the place of a 

 heavenly body or the time of a celestial phenomenon is called by 

 astronomers an equation, and as the time required by light to traverse 

 the semidiameter of the earth's orbit first i^resented itself in the guise of 

 a correction to the computed times of the eclipses of Jupiter's satellites, 

 it has received the name of the light equation. The earth's orbit being 

 interior to that of Jupiter, and both having the sun for their center, it 

 is evident that the distances between the two planets must vary from 

 the sum to the diflerence of the radii of their respective orbits, and the 

 time required by light to travel from one planet to the other must vary 

 proportionately. Consequently, if the observed times of the eclipses 

 of Jupiter's satellites are compared with the times computed upon the 

 assumption that the two planets are always separated by their mean 

 distance, it will be found that the eclipses occur too early when the 

 earth is at less than its niean distance from Jupiter, and too late when 

 it is farther oft", and from large numbers of such observations the value 

 of the light equation has been deduced. 



The combination of the motion of light through our atmosphere with 

 the orbital motion of the earth gives rise to the annual aberration, all 

 the jihases of which are comj^uted from its maximum value, commonly 

 called the constant of aberration. There is also a diurnal aberration 

 due to the rotation of the earth on its axis, but that is quite small and 

 does not concern us this evening. When aberration was discovered 

 the corpuscular theory of light was in vogue, and it ottered a charm- 

 ingly simxile explanation of the whole phenomenon. The hypothetical 



