ON THE NATURE OF LIGHT AND COLOURS. 367 



completely explicable from the general principle, of the interference of the 

 two portions encroaching perpendicularly on the shadow. (Plate XXX. 

 Fig. 445.) 



But the most obvious of all the appearances of this kind is that of the 

 fringes which are usually seen beyond the termination of any shadow, 

 formed in a beam of light, admitted through a small aperture : in white 

 light three of these fringes are usually visible, and sometimes four ; but 

 in light of one colour only, their number is greater ; and they are always 

 much narrower as they are remoter from the shadow. Their origin is 

 easity deduced from the interference of the direct light with a portion of 

 light reflected from the margin of the object which produces them, the 

 obliquity of its incidence causing a reflection so copious as to exhibit a 

 visible effect, however narrow that margin may be ; the fringes are, how- 

 ever, rendered more obvious as the quantity of this reflected light is 

 greater. Upon this theory it follows that the distance of the first dark 

 fringe from the shadow should be half as great as that of the fourth, the 

 difference of the lengths of the different paths of the light being as the 

 squares of those distances ; and the experiment precisely confirms this calcu- 

 lation, with the same slight correction only as is required in all other cases ; 

 the distances of the first fringes being always a little increased. It may 

 also be observed, that the extent of the shadow itself is always augmented, 

 and nearly in an equal degree with that of the fringes : the reason of this 

 circumstance appears to be the gradual loss of light at the edges of every 

 separate beam, which is so strongly analogous to the phenomena visible in 

 waves of water. The same cause may also perhaps have some effect in 

 producing the general modification or correction of the place of the first 

 fringes, although it appears to be scarcely sufficient for explaining the 

 whole of 'it. (Plate XXX. Fig. 446.) 



A still more common and convenient method of exhibiting the effects of 

 the mutual interference of light, is afforded us by the colours of the thin 

 plates of transparent substances. The lights are here derived from the 

 successive partial reflections produced by the upper and under surface of 

 the plate, or when the plate is viewed by transmitted light, from the direct 

 beam which is simply refracted, and that portion of it which is twice [or 

 more times] reflected within the plate. The appearance in the latter case 

 is much less striking than in the former, because the light thus affected is 

 only a small portion of the whole beam, with which it is mixed ; while in 

 the former the two reflected portions are nearly of equal intensity, and may 

 be separated from all other light tending to overpower them. In both 

 cases, when the plate is gradually reduced in thickness to an extremely 

 thin edge, the order of colours may be precisely the same as in the stripes 

 and coronae already described ; their distance only varying when the 

 surfaces of the plate, instead of being plane, are concave, as it frequently 

 happens in such experiments. The scale of an oxid, which is often 

 formed by the effect of heat on the surface of a metal, in particular of 

 h'on, affords us an example of such a series formed in reflected light : this 

 scale is at first inconceivably thin, and destroys none of the light reflected, 

 it soon, however, begins to be of a dull yellow, which changes to red, and 



