162 INTERFERENCE OF LIGHT. SECT. XX- 



tween the 0-0000258th and the 0-0000157th part of an 

 inch. Similar phenomena may be seen by viewing 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 alter- 

 nate black and brightly colored 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 ac- 

 count of the heterogeneous nature of white light, the 

 black lines alternate with vivid stripes or fringes of pris- 

 matic colors, arising from the superposition of systems 

 of alternate black lines and lines of each homogeneous 

 color. That the alternation of black lines and colored 

 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 inter- 

 rupted. 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 circumstances whatever ; while on 

 the contrary, two opposing motions may, and it is im- 

 possible not to be struck with the perfect similarity be- 

 tween the interferences of small undulations of air or of 

 water and the preceding phenomena. The analogy is 

 indeed so perfect, that philosophers of the highest au- 

 thority concur in the supposition that the celestial regions 

 are filled with an extremely rare, imponderable, and 



