530 Prof. Lovering on the Velocity of Light 



a chronometric wheelwork to move the disc. It is a remarkable 

 piece of clockwork, which solves in an elegant manner the pro- 

 blem of uniform motion in the particular case in which there is no 

 work to be done. The success is so complete that it is my daily 

 experience to launch the mirror with 400 turns a second, and see 

 the two pieces of apparatus march within yxnroir near ty °f accordance 

 during whole minutes. 



"Notwithstanding the assurance I had gained in the measure- 

 ment of time, I was surprised at proving, in my results, discordances 

 which were out of proportion to the precision of my means of measur- 

 ing. After long research, I discovered the source of error in the 

 micrometer, which did not allow of the degree of accuracy willingly 

 attributed to it. To meet this difficulty, I have introduced into the 

 system of observation a modification which amounts simply to a 

 change of the variable. Instead of measuring micrometrically the 

 deviation, I adopt for it a definite value in advance — suppose seven- 

 tenths of a millimetre, or seven entire parts of the image ; and I seek 

 by experiment to find the distance between the mark and the turn- 

 ing mirror necessary to produce this deviation : the measures extend- 

 ing over a length of about a metre, the last fractions have a magni- 

 tude directly visible, and leave no room for error. 



" By this means the apparatus has been purged of the principal 

 cause of uncertainty ; henceforth the results accorded, within the 

 limits of errors of observation, and the means are settled in such a 

 way that I am able to assign confidently the new number which 

 appears to me to express nearly the velocity of light in space, viz. 

 298,000 kilometres in a second of mean time." 



This value, reduced to statute miles, shows that the velocity 

 of light is 185,177 miles in a second; which is less by 6336 

 miles than the velocity for light usually admitted into science, 

 viz. the velocity obtained from the aberration of light. This 

 discrepancy between the results of experiment and that of the 

 astronomical determination which comes nearest to it, is three 

 times greater than the variation between the velocity deduced 

 from aberration and that derived from eclipses. 



Eoucault states that the extreme difference of the results of 

 various trials amounted to only T J^ of the whole quantity, and 

 that the mean result can be trusted to the fraction of -g-J-Q-. 

 Moreover the aberration of 20 ,/# 45 adopted by astronomers 

 cannot be supposed at fault by more than y^ of the whole. 

 Neither the velocity by Foucault's experiments nor the value of 

 aberration can be charged with a possible error of 3 per cent., 

 or of any error approaching to this large discrepancy. How is 

 the new velocity of light to be reconciled with the old value of 

 aberration ? I have said that aberration establishes only the ratio 

 between the velocity of light and the velocity of the earth. If 

 this ratio cannot be tampered with, and if one term of it (the 

 yelocity of light) must be diminished by 3 per cent, to suit Pou- 



