204 THE PRINCIPLES OF SCIENCE. 



Mr. E. J. Stone having re-discussed those observations d 

 found that grave oversights had been made in the calcu- 

 lations, which being corrected would alter the estimate 

 of parallax to 8"'gi, a quantity in such comparatively 

 close accordance with the other results that astronomers 

 did not hesitate at once to reduce their estimate of the 

 sun's mean distance from 95,274,000 to 91,771,000 miles, 

 although this alteration involved a corresponding correc- 

 tion in the assumed magnitudes and distances of most of 

 the heavenly bodies. The final decision of this question 

 of the ratio between the earth and the visible universe, so 

 far as it can be decided in the present century, must be 

 made at the approaching transits of Yenus in 1874 and 

 1882. 



In this important and interesting question the theo- 

 retical relations between the velocity of light, the constant 

 of aberration, the sun's parallax, and the sun's mean dis- 

 tance, are of the simplest character, and can hardly be 

 open to any doubt, so that the only doubt was as to which 

 result of observation was the most reliable. Eventually the 

 chief discrepancy was found to arise from misapprehension 

 in the reduction of observations, but we have a satisfactory 

 example of the value of different methods of estimation 

 in leading to the detection of a serious error. Is it not 

 surprising that Foucault by measuring the velocity of light 

 when passing through the space of a few yards, should 

 lead the way to a change in our estimates of the magni- 

 tude of the whole universe ? 



Selection of the best Mode of Measurement. 



When we have once obtained a command over a question 

 of physical science by comprehending the theory of the 



d ' Monthly Notices of the Koyal Astronomical Society,' vol. xxviii. 

 p. 264. 



