1865.] BRITISH ASSOCIATION. 323 



and mirrors, of which traces appear in Greek and Roman writers (c), 

 there had been even the first Galilean or the smallest Newtonian 

 telescope in the hands of Hipparchus, Eratosthenes, or Ptolemy, 

 would it have been left to their remote successors to be still strug- 

 gling with the elements of physical astronomy, and waiting with 

 impatience till another quarter of a century shall have rolled away? 

 and given us one more good chance of measuring the distance of 

 the sun by the transit of Venus ? Had such instruments as 

 Wheatstone's chronoscope been invented, would it have been left 

 to Foucault to condense into his own apartment an experimental 

 proof of the velocity of light, and within a tract of thirty feet to 

 determine the rate of its movement through all the vast planetary 

 space of millions and thousands of millions of miles, more exactly 

 than had been inferred by astronomers from observations of the 

 satellites of Jupiter (d,) ? By this experiment the velocity of light 

 appears to be less — sensibly less— than was previously admitted; 

 and this conclusion is of the highest interest. For, as by assum- 

 ing too long a radius for the orbit of Jupiter the calculated rate of 

 light-movement was too great ; so now, by employing the more exact 

 rate and the same measures of time, we can correct the esti- 

 mated distance of Jupiter and all the other planets from the sun» 

 We have, in fact, a really independent measure of planetary space; 

 and it concurs with observations of the parallax of Mars, in re- 

 quiring a considerable reduction of the assumed diameters of the 

 planetary paths. The distance of the earth from the sun must be 

 reduced from above ninety-five to less than ninety-three millions of 

 miles, and by this scale the other space measures of the solar 



(c) The effect of lenses or globes of glass or crystal (va\os) in collect- 

 ing the solar rays to a point, are familiarly referred to by Aristophanes 

 in the Nubes, *766 ; and the ornamental use of convex and concave re- 

 flectors is known by the curious discussions in the Fourth Book of 

 Lucretius. 



(d) Fizeau performed experiments on the velocity of light between 

 Suresnes and the Butte Montmartre, by means of the oxyhydrogen light, 

 reflected back in its own path. The space was 28,324ft. Engl. Twice 



1 



this distance was traversed in of a second=167,528 geogr. miles 



18,000 

 in a second. From observations of Jupiter's satellites, Delambre 

 inferred 167,976 miles, Struve 166,096. The experiment of M. Foucault 

 gives 298,000,000 metres=160,920 geogr. miles. 



