188 INTERFERENCE OF LIGHT. [SECT. xx. 



alone would produce. If violet light be employed, the dif- 

 ference in the lengths of the two beams must be equal to the 

 00000157th part of an inch, in order to produce the same 

 phenomena ; and, for the other colours, the difference must be 

 intermediate between the 0'0000258th and the 0-0000157th 

 part of an inch. Similar phenomena may be seen by view- 

 ing the flame of a candle through two very fine slits in a card 

 extremely near to one another (N. 193) ; or by admitting the 

 sun's light into a dark room through a pin-hole about the 

 fortieth of an inch in diameter, receiving the image on a 

 sheet of white paper, and holding a slender wire in the light. 

 Its shadow will be found to consist of a bright white bar or 

 stripe in the middle, with a series of alternate black and 

 brightly coloured stripes on each side. The rays which bend 

 round the wire in two streams are of equal lengths in the 

 middle stripe ; it is consequently doubly bright from their 

 combined effect ; but the rays which fall on the paper on 

 each side of the bright stripe, being of such unequal lengths 

 as to destroy one another, form black lines. On each side of 

 these black lines the rays are again of such lengths as to 

 combine to form bright stripes, and so on alternately till the 

 light is too faint to be visible. When any homogeneous light 

 is used, such as red, the alternations are only black and red ; 

 but, on account of the heterogeneous nature of white light, the 

 black lines alternate with vivid stripes or fringes of prismatic 

 colours, arising from the superposition of systems of alternate 

 black lines and lines of each homogeneous colour. That the 

 alternation of black lines and coloured fringes actually does 

 arise from the mixture of the two streams of light which flow 

 round the wire, is proved by their vanishing the instant one of 

 the streams is interrupted. It may therefore be concluded, 

 as often as these stripes of light and darkness occur, that they 

 are owing to the rays combining at certain intervals to pro- 

 duce a joint effect, and at others to extinguish one another. 

 Now it is contrary to all our ideas of matter to suppose that 

 two particles of it should annihilate one another under any 



