Page:Proceedings of the Royal Society of London Vol 1.djvu/113

Rh lustrates it still further, by applying it to some new distinct cases relating 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 phaenomenon 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 eﬂ’ect similar to this are ascribed the coloured atmospherical halos, substi- tuting to the above ﬁbra 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 formerly advanced. He supposes that every refractive medium transmits 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 successive separation, the one having preceded the other by a very minute 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