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 hieroglyphies 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 unnaturally 
transmitted the narrow beam of light through éwo 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 oprque 
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 opague 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 
