Page:Proceedings of the Royal Society of London Vol 1.djvu/111

Rh from greatest to least, will become at once manifest by mere inspec- tion. Lastly, a table is given containing a series of substances ar- ranged according to their refractive powers, chieﬂy deduced from ex- periments made according to the method here described; some to which the machine for measurement would not apply, being ob- tained by other means, or borrowed from other authors.

The second part of the paper treats of the dispersion of light. The principles and observations on which the inductions here given chieﬂy depend, are these :—When a glass prism is placed in contact with water, and brought near the eye, in such a position that it reﬂects the light from the window, the extent of perfect reﬂection is seen to be bounded by a fringe of the prismatic colours in the order of their tefrangibility. But it may happen that two media which refract un- equally at the same incidence, may disperse equally at that incidence; and, under these circumstances, a pencil of rays paSsing from one of these media into the other, will be refracted without dispersion of its colours. The boundary of prismatic reﬂection will then be found a well deﬁned line, free from colour, if the surface at which the reﬂected light emerges from the prism be at right angles to its course. More- over, when the disparity of the dispersive powers of the media is still greater, it may also happen that the usual order of prismatic colours will be reversed, and then the red, or least refrangible ray, will appear strongest and lowest in the fringe, unless the colours so produced are counteracted by refraction at their emergence from the prism.

This doctrine is illustrated by examples of various, both simple and compound, substances, and especially by the effect of metallic solu- tions differently diluted in less dispersive media. Having compared several of these, each diluted till the limit of reﬂection appeared void of colour when in contact with a rectangular piece of plate-glass, he deduced thence a table of their refractive powers, in that state of di- lution in which the eye could discern the disappearance of colour.

He likewise made experiments on dispersion by means of wedges, in the manner practised by .Mr. Dollond, Dr. Blair, and others; and has reduced the substances thus examined into a second table, ar- ranged according to the excess of their effect on violet above red light, at a given angle of deviation. By comparing this with the preceding table, it appears how little correspondence there is between them, and consequently how numerous are the combinations by means of which a pencil of rays that passes through two media, may be made to deviate Without dispersion of its colours.

At the close of this paper the author remarks, that the colours into which a beam of white light is separable by refraction, appear to him to be neither seven, as they are usually seen in the rainbow, nor reducible to three, as some persons have conceived; but that by employing a very narrow pencil of light, four primary divisions of the prismatic spectrum may be seen, with a degree of distinctness which, he believes, has never been described nor observed before. These colours are red, yellowish green, blue, and violet, in the proportion nearly as the numbers 16, 23, 36, 25.