22 The Theory of the Aether 



ultimately made by an astronomer. It was observed in 1675 

 by Olof Roemer* (b. 1644, d. 1710) that the eclipses of the first 

 satellites of Jupiter were apparently affected by an unknown 

 disturbing cause ; the time of the occurrence of the phenomenon 

 was retarded when the earth and Jupiter, in the course of their 

 orbital motions, happened to be most remote from each other, 

 and accelerated in the contrary case. Eoemer explained this 

 by supposing that light requires a finite time for its pro- 

 pagation from the satellite to the earth ; and by observations of 

 eclipses, he calculated the interval required for its passage from 

 the sun to the earth (the light-equation, as it is called) to be 

 11 minutes, f 



Shortly after Roemer's discovery, the wave-theory of light 

 was greatly improved and extended by Christiaan Huygens 

 (b. 1629, d. 1695). Huygens, who at the time was living in 

 Paris, communicated his results in 1678 to Cassini, Eoemer, 

 De la Hire, and the other physicists of the French Academy, 

 and prepared a manuscript of considerable length on the subject. 

 This he proposed to translate into Latin, and to publish in that 

 language together with a treatise on the Optics of Telescopes ; 

 but the work of translation making little progress, after a delay 

 of twelve years, he decided to print the work on wave-theory 

 in its original form. In 1690 it appeared at Ley den, J under 

 the title Traite de la lumiere ou sont expliquees les causes de ce 

 qui luy arrive dans la reflexion et dans la refraction. Et parti- 



*Mem. de 1'Acad. x. (1666-1699), p. 575. 



t It was soon recognized that Roemer's value was too large ; and the 

 astronomers of the succeeding half-century reduced it to 7 minutes. Delambre, 

 by an investigation whose details appear to have been completely destroyed, 

 published in 1817 the value 493 -2 s , from a discussion of eclipses of Jupiter's 

 satellites during the previous 150 years. Glasenapp, in an inaugural dissertation 

 published in 1875, discussed the eclipses of the first satellite between 1848 and 

 1870, and derived, by different assumptions, values between 496 s and 501 s , the 

 most probable value being 500-8 8 . Sampson, in 1909, derived 498'64 S from his 

 own readings of the Harvard Observations, and 498'79 S from the Harvard readings, 

 with probable errors of about + 0'02". The inequalities of Jupiter's surface give 

 rise to some difficulty in exact determinations. 



% Huygens had by this time returned to Holland. 



