366 LECTURE XXXIX. 



will still appear ; and in this manner the colours of small fibres are pro- 

 bably 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 belonging 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 dis- 

 tances 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, may be exhibited. (Plate XXX. Fig. 44-fT) 



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

 rates the parallel slits from each other, their external margins will still 

 continue to exhibit Jhe effects of diffracted 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 concerned in the appearance of fringes, wherever their 

 whole 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 obliquity, 

 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 appear- 

 ance of colour in the usual manner : thus when the surface is so situated, 

 that the image of the luminous point would be seen in it by regular reflec- 

 tion, the difference will vanish, and the light will remain perfectly white, 

 but in other cases various colours will appear, according to the degree of 

 obliquity. These colours may easily be seen, in an irregular form, by 

 looking at any metal, coarsely polished, in the sunshine ; but they be- 

 come more distinct and conspicuous, when a number of fine lines of 

 equal strength are drawn parallel to each other, so as to conspire in their 

 effects.* 



It sometimes happens that an object, of which a shadow is formed in a 

 beam of light, admitted through a small aperture, is not terminated by 

 parallel sides ; thus the two portions of light, which are diffracted from 

 two sides of an object, at right angles with each other, frequently form 

 a short series of curved fringes within the shadow, situated on each side 

 of the diagonal, which were first observed by Grimaldi,t and which are 



* Young's Introduction to Medical Literature, 1813, p. 559. 



f* Physico-Mathesis de I^imine, Coloribus et Iride, Bonon. 1665. 



