466 LECTURE XXXIX. 



tinues to produce the same fringes as before, but they arc not easily visible, 

 except within the extent of its shadow, being overpowered in other parts by 

 a stronger light; but if the light thus diffracted be allowed to fall on the 

 eye, either within the shadow, or in its neighbourhood, the stripes will still 

 appear; and in this manner the colours of small fibres are probably formed. 

 Hence if a collection of equal fibres, for example a lock of wool, be held before 

 the eye when we look at a luminous object, the series of stripes belono-ino- 

 to each fibre combine their effects, in such a manner, as to be converted into 

 circular fringes or coronae. This is probably the origin of the coloured 

 circles or coronae sometimes seen round the sun and moon, two or three of 

 them appearing together, nearly at equal distances from each other and from 

 the luminary, the internal ones being, however, like the stripes, a little 

 dilated. It is only necessary that the air should be loaded with globules of 

 moisture, nearly of equal size among themselves, not much exceeding one 

 two thousandth of an inch in diameter, in order that a series of such coronae, 

 at the distance of two or three degrees from each other, maybe exhibited. 

 (Plate XXX. Fig. 444.) 



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If, on the other hand, we remove the portion of the screen which separates 

 the parallel slits from each other, their external margins will still continue to ex- 

 hibit the effects of di.ffracted light in the shadow on each' side; and the 

 experiment will assume the form of those which were made by Newton on 

 the light passing between the edges of two knives, brought very nearly into 

 contact; although some of these experiments appear to show the influence of 

 a portion of light reflected by a remoter part of the polished edge of the 

 knives, which indeed must unavoidably constitute a part of the light con- 

 cerned in the appearance of fringes, wherever their Avhole breadth exceeds 

 that of the aperture, or of the shadow of the fibre. 



The edges of two knives, placed very near each other, may represent the 

 opposite margins of a minute furrow, cut in the surface of a polished sub- 

 stance of any kind, which, when viewed with different degrees of obli- 

 quity, present a series of colours nearly resembling those which are exhibited 

 within the shadows of the knives: in this case, however, the paths of the 

 two portions of light before their incidence are also to be considered, and 

 the whole difference of these paths will be found to determine the appearance of 



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