312 THOMAS YOUNG. 



refraction produces by the destruction of light resulting 

 from the interference of rays; but it is to Young that the 

 honour belongs of having opened the way ; it was he 

 who was the first to decypher these hieroglyphics of 

 optics.* 



* It has been well observed that simplicity is not always a fruit 

 of the first growth, and accordingly some of the earliest of Young's 

 researches were complicated by unnecessary conditions. Thus, to 

 exhibit the effect of two rays interfering, he at first not unnatural ly 

 transmitted the narrow beam of light through two small apertures near 

 together. In point of fact, though the real effect is here seen, it is 

 mixed up with others of a more complex kind. The narrow apertures 

 each exhibited coloured fringes, in addition to the interference stripes 

 seen between them. The coloured fringes of apertures (unless very 

 wide) are distinct from those formed by one external edge of an opaque 

 body ; the light from each side conspires to the effect in a somewhat 

 complex manner. If the aperture be otherwise than long with parallel 

 sides, the phenomenon becomes still more complex, and the calcula- 

 tion difficult; few such cases have ever yet been solved, and some such 

 cases have been dwelt upon as formidable objections to the theory; 

 they are simply cases to which the formula, from its mathematical 

 difficulties, has not yet been extended. 



In all these cases of diffraction an opaque body was used, and it 

 might still be suspected that some action of the edge of that body might 

 be concerned in the result. Numerous experiments of Maraldi, Dutour, 

 Biot, and others, were directed to the investigation of this point. Biot 

 showed that an opaque body was not necessary, inasmuch as the edge 

 of a plate of glass, or even the bounding line of two faces of a glass cut 

 at a slight inclination to each other, gave the same fringes; indeed, 

 Newton also had noticed something of the kind. Haldat varied the 

 conditions of the edge in every conceivable way, whether of form or 

 nature, by the influence of magnetism, galvanism, electricity, or tem- 

 perature from freezing to a red heat, without producing the slightest 

 difference in the fringes; a result which it would be impossible to con- 

 ceive compatible with any idea of an atmosphere of attraction or repul- 

 sion surrounding the edge. 



Again, though we have given the explanation of the external fringes 

 in its simple and correct form, yet both Young and Fresnel failed in 

 the first instance to see it in that light, both believing that the reflexion 

 of a portion of rays from the edge of the opaque body was mainly con- 

 cerned in producing the interference. Subsequent experiments showed 



