3l8 ON LIGHT. 



of stars when viewed through telescojoes of great magni- 

 fying power, and with apertures either of tlie usual 

 circular form, or of forms otherwise varied for tliis ex- 

 press purpose. And though last in our order of enumer- 

 ation, by far the most important and instructive as 

 elucidatory of the principle of explanation, applicable to 

 all the phrenomena of this class ; tlie coloured fringes 

 seen to follow the outlines of shadows wlien thrown by 

 a light emanating from an extremely small but intensely 

 luminous j^oint. With these, therefore, we shall begin, 



(102.) It was objected to the undulatory theory of 

 light by Newton himself, that sound, to which that 

 theory assimilates it, spreads from an aperture through 

 which it is transmitted, or round the edge of an inter- 

 cepting screen of any kind, equally in all directions ; and 

 thus, were the analogy exact, there could be no shadows. 

 The objection is founded partly on an imperfect state- 

 ment of the fact, and partly on omitting to allow for pos- 

 sible differences in the natures of the conveying media, 

 and in the modes of vibratory motion conveyed. Every 

 one is faniiliar with the sudden outbreak of sound from 

 a railway train heard at a great distance when it emerges 

 from a cutting, or turns the corner of a wall or of a hill. 

 Sound is propagated through water with greater sharp- 

 ness, velocity, and distinctness, than through air. But 

 an obstacle interposed under water, as a projecting pier, 

 or a rock, cuts off the rays of sou/id, as appears from 

 direct experiment, with much greater definiteness than 

 in air, and casts, so to speak, an evident acoustic shadow. 

 Nor will it ai')pear at all surprising that an effect of this 



