_u_ (_Tab. 26._) the cock, that fastens the pinion G, to the inside of the wheel A. _w_ (_Tab. 26._) the cock, that fastens the pinion H on the outside of the wheel A. _x_ (_Tab. 27._) the arbor, on which the pinions G and H are fixed. _y_ (_Tab. 26._) a spring, that keeps the click _a_ in its place. L (_Tab. 26._) a frame-plate with the centers marked. _z_ The opposite hole enlarged, to admit the pinion _c_ to pass through. XCVIII. _An Account of some Experiments concerning the different Refrangibility of Light. By Mr._ John Dollond. _With a Letter from_ James Short, _M. A. F.R.S. Acad. Reg. Suec. Soc._ [Read June 8, 1758.] _To the Rev. Dr._ Birch, _Secret. R. S._ Dear Sir, I Have received the inclosed paper from Mr. Dollond, which he desires may be laid before the Royal Society. It contains the theory of correcting the errors arising from the different refrangibility of the rays of light in the object-glasses of refracting telescopes; and I have found, upon examination, that telescopes made according to this theory are intirely free from colours, and are as distinct as reflecting telescopes. I am, Dear Sir, Your most obedient humble Servant, Ja. Short. Surrey-street, 8th June, 1758. IT is well known, that a ray of light, refracted by passing thro’ mediums of different densities, is at the same time proportionally divided or spread into a number of parts, commonly called homogeneal rays, each of a different colour; and that these, after refraction, proceed diverging; a proof, that they are differently refracted, and that light consists of parts that differ in degrees of refrangibility. Every ray of light passing from a rarer into a denser medium, is refracted towards the perpendicular; but from a denser into a rarer one, from the perpendicular; and the sines of the angles of incidence and refraction are in a given ratio. But light consisting of parts, which are differently refrangible, each part of an original or compound ray has a ratio peculiar to itself; and therefore the more a heterogene ray is refracted, the more will the colours diverge, since the ratios of the sines of the homogene rays are constant; and equal refractions produce equal divergencies. That this is the case when light is refracted by one given medium only, as suppose any particular sort of glass, is out of all dispute, being indeed self-evident; but that the divergency of the colours will be the same under equal refractions, whatsoever mediums the light may be refracted by, tho’ generally supposed, does not appear quite so clearly.