194 On the Measurement of Minute Particles. [Sept. 



light bent by diffraction round the remoter side of a fibre : for 

 thi6 light always co-operates in the first instance with that which 

 is reflected from the nearer side. The extent of the central 

 white light is indeed so great, that all the coloured appearances 

 may almost be considered as beginning at such a distance, that 

 the first dark space is exactly where the simple calculation would 

 lead us to expect the white ; since the value of the unit of the 

 eriometer ought to be, according to this calculation, about 

 % i x 000 of an inch, instead of •■ „ \ - 00 ', and indeed this value 

 agrees very accurately with experiment, where the two portions 

 of light concerned are exactly in similar circumstances; as may 

 be observed in some of the parallel lines drawn on glass in Mr. 

 Coventry's micrometers, probably where they happen to be 

 single, for in general they are double, and exhibit colours cor- 

 responding to an interval much smaller than their regular dis- 

 tance : but in some parts we may observe colours exactly cor- 

 responding to their distance, for instance, to -j-J-g- of an inch, 

 according to the simple principle of considering each unit as 

 equal to about the 43000th of an inch. Hence it seems that 

 the necessity of a correction depends on the different state of the 

 lights reflected from one side of a fibre, and diffracted round its 

 opposite side, and that when they proceed in a similar manner 

 from two neighbouring parallel lines, the necessity no longer 

 exists. What may be the cause of this irregularity, will perhaps 

 be understood when we understand the cause of the singular 

 phenomena of oblique reflection discovered by Mr. Malus, and 

 we have no reason to expect to understand it before. 



VII. Glories. 



1 have had an opportunity of ascertaining, that the clouds 

 which exhibit the white and coloured circles, sometimes deno- 

 minated glories, are certainly not composed of icy particles ; 

 and I have succeeded in deducing an explanation of these phe- 

 nomena from the same laws, which are capable of being applied 

 to so many other cases of physical optics. In the theory of su- 

 pernumerary rainbows, (Nat. Phil. I. 471, PI. 30, Fig. 451, II. 

 643,) I have observed that the breadth of each bow must be the 

 greater as the drops which afford it are smaller ; and by consi- 

 dering the coloured figure, in which their production is analyzed, 

 it will be obvious, that if we suppose the coloured stripes ex- 

 tremely broad, they will coincide in such a manner in one part 

 as to form a white bow : the red, which projects beyond the 

 rest, being always broadest, so that if all the stripes be sup- 

 posed to expand, while they preserve their comparative magni- 

 tude, the middle of the red may coincide with the middle of the 

 blue; and it will appear on calculation that a white bow will 

 be formed, a few degrees within the usual place of the coloured 



