10 



SCIENTIFIC THOUGHT. 



erate shortly the different suggestions which Fresnel 

 assimilated and worked up into his celebrated physical 

 theory of light. 



That light consisted in the motion of something was 

 in the beginning of the nineteenth century a generally 

 accepted notion among natural philosophers. It had 

 been so ever since Olaus Eomer 1 in the seventeenth 

 century, from the observation of the hitherto unexplained 

 delay in the disappearance of Jupiter's satellites during 

 eclipses, had inferred, and Bradley 2 had later on con- 



1 The moons of Jupiter, of which 

 two are visible to the naked eye, 

 were clearly seen and described as 

 one of the first discoveries with his 

 telescope by Galileo in 1610, and 

 published in his ' Sidereus Nuncius.' 

 Owing to their continual and rapid 

 change of position and their fre- 

 quent eclipses, they were very soon 

 considered to furnish a valuable 

 means of determining the longi- 

 tude at sea, and were repeatedly 

 and very minutely observed. In 

 the course of such observations by 

 Cassini and Homer at Paris, the 

 latter found, in 1675, that the period 

 of occultation of the nearest moon 

 varied. This variation he traced 

 to the fact that the earth was 

 moving towards or away from 

 Jupiter. If light takes time to 

 travel, the visibility of the pheno- 

 menon is necessarily thus antici- 

 pated or postponed. This was the 

 first occasion on which data for the 

 calculation of the velocity of light 

 were forthcoming ; the terrestrial 

 experiments of Galileo having been 

 inconclusive. Homer's explanation 

 and calculation were accepted by 

 most astronomers ; they were con- 

 firmed by 



2 the phenomenon of aberration, 

 discovered by Bradley. It is ana- 

 logous to the observation we can 



make in a moving railway train 

 if it rains ; the drops at the win- 

 dow, though they be descending 

 perpendicularly, yet appearing in 

 a slanting direction, in propor- 

 tion to the velocity of the train. 

 Both phenomena involve the mo- 

 tion of light itself and the motion 

 of the observer, who receives the 

 luminous impression and locates it 

 in space and time. The principle 

 involved in Homer's discovery was 

 later enunciated by Doppler, who 

 maintained that the very short 

 periods which belong to different 

 colours of the spectrum, according 

 to the undulatory theory, must 

 suffer (like the longer periods in 

 Romer's occultations) by the mo- 

 tion of the luminous object or of 

 the observer in the line of sight. Al- 

 though this theory was admitted 

 in acoustics, it ' took some time 

 before it was admitted in optics. 

 Bolzano, Professor of Religious Phil- 

 osophy and a colleague of Doppler 

 at Prague, foretold as early as 1842 

 the great utility of the principle, 

 and wrote : " I foresee with con- 

 fidence that use will be hereafter 

 made of it in order to solve by 

 observing the changes which the 

 colour of stars undergoes in time 

 the questions whether and in 

 which direction and with what 



