93 



lustrates it still further, by applying it to some new distinct cases re- 

 lating to the colours of fibres, and to the colours of mixed plates. 



The case respecting fibres is that of the coloured fringes produced 

 by the interposition of a hair between the luminous object and the 

 eye. Here it is observed that the fringes are larger and brighter in 

 proportion as the hair is thinner, the phenomenon being most con- 

 spicuous when a single thread of a silkworm is interposed. The cause 

 here assigned is the interference of two portions of light, one re- 

 flected from the fibre, and the other bending round its opposite side, 

 and at last coinciding nearly in direction with the former portion ; 

 hence as both portions deviate more from a rectilinear direction, the 

 difference of the length of their paths will be gradually increased, 

 and consequently produce the appearances of colour usual in such 

 cases. The rule given to calculate the difference of the paths for the 

 light least inflected, is the analogy between that difference to the 

 diameter of the fibre, which will be as the deviation of the ray at any 

 point from the rectilinear direction, to its distance from the fibre. 



When a number of fibres of the same kind, as, for instance, an 

 uniform lock of wool, are held near to the eye, we see an appear- 

 ance of halos surrounding a distant candle ; but their brilliancy, and 

 even their existence, depends on the uniformity of the dimensions of 

 the fibres, and they are larger as the fibres are smaller. To an effect 

 similar to this are ascribed the coloured atmospherical halos, substi- 

 tuting to the above fibres a number of particles of water of equal 

 dimensions, and properly situated between the luminary and the 

 eye. 



Speaking of the colours of mixed plates in looking at a candle 

 through two pieces of plate-glass with a little moisture between them, 

 we are told that the fringes here produced are the effect of moisture, 

 intermixed with portions of air, exhibiting an appearance similar to 

 dew. Here the light transmitted through the water, moving in it 

 with a velocity different from that of the light passing through the 

 interstices filled only with air, the two portions, it is said, will inter- 

 fere with each other, and produce the effect of colours according to 

 the general law. 



In further applying this general law, the author found it impossible 

 to avoid another supposition, which is a part of the undulatory theory 

 he defended in his former paper ; namely, that the velocity of light 

 is the greater the rarer the medium : and he suggests an idea, which 

 appears to him to lead to an explanation of the dispersion of colours 

 by refraction, more simple and satisfactory than that which he for- 

 merly advanced. He supposes that every refractive medium trans- 

 mits the undulations constituting light in two separate portions, one 

 passing through its ultimate particles, and the other through its 

 pores ; and that these portions re-unite continually after each suc- 

 cessive separation, the one having preceded the other by a very mi- 

 nute but constant interval, depending on the regular arrangement of 

 the particles of a homogeneous medium. Having briefly discoursed 

 on the application of this doctrine, he concludes by mentioning some 



