VELOCITY OF LIGHT. 413 



occultation a continually increasing time for travelling 

 through the increasing distance between the two 

 planets ; hence the eclipses appear to be retarded, and 

 the interval between one and the next is now 42h. 

 28m. 49-8s. 



The retardation or acceleration of the eclipses of 

 Jupiter's satellite may be calculated, as has been shown, 

 from the known velocity of light. It is clear that the 

 preceding mode of reasoning may be reversed, and that 

 from the difference between the observed and the cal- 

 culated times of the eclipses the velocity of light may 

 be deduced. This was indeed the first method by 

 which it was found that light requires time in order 

 to pass from one point of space to another, and it was 

 thus calculated that it passes in one second through 

 190,000 miles ; subsequent observations have, however, 

 shown that the distances between the planets were 

 ormerly taken to be somewhat greater than they really 

 are, and the velocity of light is now known to be about 

 186,000 miles per second. 



Air and other gaseous substances, except those which 

 are coloured, permit light to pass through them without 

 hindrance, unless they are rendered cloudy by suspended 

 dust or fog. If air is very pure, as is the case on high 

 mountains, light passes through distances of many miles 

 without being sensibly diminished. Most liquids also 

 are transparent, but not in so perfect a degree as gases ; 

 many liquids, even those which seem quite transparent, 

 appear coloured when viewed in thick strata ; thus a 

 layer of pure water 2 metres thick appears of a bluish 

 tint. Of solid bodies only a comparatively limited 

 number permit light to pass readily for example, 



