Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/120

 this angle is to the sine of the visible inclination of the object to the line in which the eye is moving, as the velocity of the eye is to the velocity of light. Observations such as Bradley's will therefore enable us to deduce the ratio of the mean orbital velocity of the earth to the velocity of light, or, as it is called, the constant of aberration; from its value Bradley calculated that light is propagated from the sun to the earth in 8 minutes 12 seconds, which, as he remarked, "is as it were a Mean betwixt what had at different times been determined from the eclipses of Jupiter's satellites."

With the exception of Bradley's discovery, which was. primarily astronomical rather than optical, the eighteenth century was decidedly barren, as regards both the experimental and the theoretical investigation of light; in curious contrast. to the brilliance of its record in respect of electrical researches. But some attention must be given to a suggestive study of the aether, for which the younger John Bernoulli (b. 1710, d. 1790). was in 1736 awarded the prize of the French Academy, His: ideas seem to have been originally suggested by an attempt