116 NEW MEASUEEMENTS OF DISTANCE OF SUN. 



No^Y, the semiamplitude of the oscillation is only "0?) second, a 

 quantity so small that one can not but feel doubts as to its reality. 

 At first I was myself inclined to disbelieve ; but when a new distribu- 

 tion of the residuals, starting from a different zero of time, gave a 

 similar periodicity, it began to look as if there were something in it. 

 The more I look at it the more I believe that it is a reality, and that 

 the photographs have shown themselves accurate enough to detect an 

 inequality of '03 second, corresponding to a shift of 5 miles at a 

 distance of 25,000,000 miles — by far the smallest inequality in the 

 motion of a planet ever brought to light by observation. 



It is this circumstance that encourages one to believe in the accu- 

 racy of the photographs. There are altogether 10,000 separate expo- 

 sures of the planet which will within the next few years be meas- 

 ured and made available for discussion. If 300 give a P. E. of 

 0-005 second, what will 10,000 give, added to G,000 or 7,000 sets of 



Fig. 7. — Residuals grouped in (juarters of the half period. 



visual observations? It would be going too far to apply the simple 

 rules of probability and say a good deal less than "001 second. But 

 I fully believe that if this great array of observations is ever sub- 

 mitted to complete discussion the probable error of the result will not 

 be much above '001 second. And supposing that it should support 

 Avith its greater weight the value 8 -80 seconds which has been 

 assailed, I believe that we should be justified in saying that the solar 

 parallax is 8 "80 seconds, and in maintaining the proposition that the 

 determination of the solar parallax is a problem of geometry and 

 celestial surveying, and that upon the sponsors of the indirect meth- 

 ods lies the onus of showing cause for their disagreement. 



This opens up an interesting prospect. Suppose that in course 

 of time there should come to be a clear and definite agreement among 

 the values found for the constant of the aberration of light, and that 

 its value was (let us say) 20*54 seconds, corresponding, as this table 

 shows, to a parallax of 8 -77 seconds, not 8 -80 seconds, on the assump- 

 tion at least that the velocity of light is exactly determined, as it 

 seems to be, and that the simple theory of aberration is correct. 



