112 ELEMENTS OF SCIENCE 



examining an exceedingly narrow line of light passing 

 through a very perfect prism. The spectrum so produced 

 appears as a band of coloured light which is continuous 

 save that it is interrupted by very numerous dark parallel 

 lines indicating so many rays which do not make their 

 existence visible. The arrangement of these dark lines 

 is different in light derived from different sources. 



As has been said, the favourite hypothesis now made 

 use of to co-ordinate the various phenomena of light, is 

 that of regular vibrations or oscillations of ether. What- 

 ever may be the absolute truth, there certainly is a 

 definite periodical action or influence of some kind which 

 takes place with amazing rapidity, but a rapidity which is 

 different in the differently coloured rays. For the sake of 

 simplicity of illustration, we may represent these period- 

 ical actions, or influences, as so many steps. It has been 

 ascertained that a violet ray takes 64,631 such steps in 

 every inch of space it traverses. Such a ray has been 

 calculated to take about 786,000,000,000,000 steps in a 

 second, while a red ray takes about 449,000,000,000,000 

 steps, and as, when unretarded by any medium, the differ- 

 ent rays advance at the same speed, the steps of the red 

 ray must be much longer than those of the violet one. 

 But different rays are retarded unequally in passing 

 through different media, and this fact explains both what 

 is called the interference of light and the phenomena of 

 iridescence. The steps taken by any two rays of light of 

 the same degree of refrangibility, are equal in length. If, 

 however, they are together to produce a more visible 

 effect than a single ray produces, they must " keep step " 

 their actions must be synchronous. This shows that 

 in each step there are two actions or influences, which 

 are opposed to each other and which we may represent 

 by those most generalised signs + and - . 



