226 LECTURES TO SCIENCE TEACHERS. 



being bent down much more than the red rays are, and 

 consequently this undulatory theory must assume that the 

 velocity of light is different for different colours in a dense 

 medium. We know that it is not so in ether or in the 

 atmosphere to any appreciable extent, for otherwise, in 

 the occultation of a star, when a star reappears from 

 behind the surface of the moon, if the violet rays travel 

 quickest, the star should first appear violet, and then the 

 other colours should gradually be added ; but such an 

 effect has never been observed. But it is quite possible 

 that in a dense medium the velocity of the violet rays may 

 be less than the velocity of the red rays. The theory has 

 only partially been worked out by the brilliant investiga- 

 tions of M. Cochet, but he founded his theory on an 

 assumption as to the nature of the ether which is quite 

 untenable ; nevertheless the nature of his results was to 

 show that the velocity of light in a dense medium would 

 depend on the wave length of light, that is to say, on its 

 colour ; and it is extremely probable that although M. 

 Cochet employed an erroneous theory of the nature of the 

 ether, that any true theory which we could work out would 

 lead to the same results as those which he got. However, 

 it is a very great desideratum to prove that the velocity of 

 light in a dense medium is dependent on the colour of the 

 light which is passing through the medium, and it is to be 

 hoped that before many years such a result may be 

 obtained. 



I think I have now described nearly all the facts which 

 have been observed in connection with the velocity of 

 light hitherto, and you will see that although it is a sub- 

 ject of very great difficulty, it has been attacked from very 

 different sides, and in all cases very concordant results 

 have been obtained ; and this does enormous credit to the 

 ingenuity of the methods employed, chiefly because these 

 methods were so entirely original. 



