168 INTERFERENCE OF LIGHT. SECT. X. 



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. 198); 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 produce 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 contaary, two opposing motions may ; and it is impos- 

 sible not to be struck with the perfect similarity between the 

 interferences of small undulations of air or of water and the pre- 

 ceding phenomena. The analogy is indeed so perfect, that 

 philosophers of the highest authority concur in the belief that 

 the celestial regions are filled with an extremely rare and highly 

 elastic medium or ether, whose particles are capable of receiving 

 the vibrations communicated to them by self-luminous bodies, 

 and of transmitting them to the optic nerves, so as to produce 

 the sensation of light. The acceleration in the mean motion of 



