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The great Principle of Nature is this, In all elastic Fluids, [[underlined]]the denser parts always press towards the rarer,[[/underlined]] that the equilibrium of of both may be restored. 2.ly [[abbreviation for "secondly"]] The Pressure, Strength, or actions of fluids in motion, upon bodies in fluids, &c. are as the [[underlined]]Activity[[/underlined]] and [[underlined]]quantity of Matter[[/underlined]] or [[underlined]]Density[[/underlined]] of the fluid: thus, by putting a small Candle under a large Receiver of an Air-pump, the air within is expanded & put into an active motion by the Candle, and so will depress the mercury in the gage, much lower than the outward surface in the vessel in which the gage is placed: but in a short time, the air passes thro' the Candle, is generated into [[underlined]]light[[/underlined]], (a much rarer fluid) and flies out of the receiver; as soon as this rarifaction does not counterballance the activity, the mercury will rise higher in the gage than in the vessel. Water Rising in the ratio of 30 to 1 to that of mercury, will be much more sensible in this experiment. W.J.

No. XCVIII. (of the Phil. Trans. Vol. 50. part. 2. & for 1758.) [[underlined]] An account of some Experiments concerning the different Refrangibility of Light. [[underlined]]By Mr. [[/underlined]]  John Dollond. [[underlined]]With a Letter from [[/underlined]] James Short, M.A.F.R.S. [[underlined]]Acad.[[/underlined]] Reg. [[underlined]]Suec. Soc.[[/underlined]]

Dear Sir,
Read June 8, 1758. I have received the inclosed paper from Mr. Dollard, 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; [[strikethrough]]made according[[/strikethrough]] 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 array 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 [[strikethrough]] thro' [[/strikethrough]] from a rarer into a denser medium, is refracted towards the perpendicular; but from a denser to a rarer one, from the perpendicular; and the sines of the angles of incidence and