en to bodies in the air-pump. By the af fiftance of telescopes bodies have been brought nearer to one another. Finally, the feveral difcoveries which Sir Ifaac Newton has made on light, are equal to the boldest things which the curiofity of man could expect, after fo many philofophical novelties. TILL Antonio de Dominis, the rainbow was confider'd as an inexplicable miracle. This philofopher guefs'd, that it was a neceffary effect of the fun and rain. Des Cartes gain'd immortal fame by his mathematical explication of this fo natural a phænomenon. He calculated the reflections and refractions of light in drops of rain; and his fagacity on this occafion was at that time look'd upon as next to divine. BUT what would he have faid had it been prov❜d to him, that he was mistaken in the nature of light; that he had not the leaft reafon to maintain that 'tis a globular body; that 'tis falfe to affert, that this matter spreading itself through the whole, waits only to be projected forward by the fun, in order to be put in action, in like manner as a long staff acts at one end when pufh'd forward by the other; that light is certainly darted by the fun; in fine, that light is tranfmitted from the fun to the earth in about feven minutes, non ball, which were not to lose tho' a canany of its velocity, velocity, cou'd not go that distance in lefs than twenty-five years? How great wou'd have been his aftonishment, had he been told, that light does not reflect directly by impinging against the solid parts of bodies, that bodies are not tranfparent when they have large pores; and that a man fhould arife, who would demonstrate all thefe paradoxes, and anatomize a fingle ray of light with more dexterity than the ablest artist diffects a human body? This man is come. Sir Ifaac Newton has demonftrated to the eye, by the bare affistance of the prism, that light is a compofition of colour'd rays, which, being united, form the white colour. A fingle ray is by him divided into seven, which all fall upon a piece of linnen, or a fheet of white paper, in their order one above the other, and at unequal distances. The firft is red, the fecond orange, the third yellow, the fourth green, the fifth blue, the fixth indigo, the feventh a violet purple. Each of these rays tranfmitted afterwards by an hundred other prifms, will never change the colour it bears; in like manner as gold, when completely purg'd from its drofs, will never change afterwards in the crucible. As a fuperabundant proof that each of these elementary rays has inherently in itfelf that which forms its colour to the eve, take a small piece of yellow wood for inftance, and fet it it in the ray of a red colour, this wood will inftantly be ting'd red; but fet it in the ray of a green colour, it affumes a green colour, and fo of all the rest. FROM what cause therefore do colours arise in nature? "Tis nothing but the difpofition of bodies to reflect the rays of a certain order, and to abforb all the reft. WHAT then is this fecret difpofition? Sir Ifaac Newton demonftrates, that 'tis nothing more than the density of the small constituent particles of which a body is compos'd. And how is this reflexion perform'd? 'Twas fuppos'd to arise from the rebounding of the rays, in the fame manner as a ball on the surface of a solid body; but this is a mistake, for Sir Isaac taught the astonish'd philofophers, that bodies are opake for no other reason, but because their pores are large; that light reflects on our eyes from the very bolom of those pores; that the smaller the pores of a body are, the more fuch a body is transparent. Thus paper, which reflects the light when dry, tranfmits it when oil'd, because the oil, by filling its pores, makes them much smaller. 'Tis there that examining the vast porofity of bodies, every particle having its pores, and every particle of thofe particles having its own; he fhews we are not certain that there is a cubic inch of folid matter matter in the universe, so far are we from conceiving what matter is. Having thus divided, as it were, light into its elements, and carried the fagacity of his difcoveries fo far, as to prove the method of diftinguishing compound colours from fuch as are primitive; he fhews, that thefe elementary rays, feparated by the prifm, are rang'd in their order for no other reafon but because they are refracted in that very order; and 'tis this property (unknown till he difcover'd it) of breaking or fplitting in this proportion; 'tis this unequal refraction of rays, this power of refracting the red lefs than the orange colour, &c. which he calls the different refrangibility. The most reflexible rays are the most refrangible, and from hence he evinces that the fame power is the cause both of the reflexion and refraction of light. BUT all thefe wonders are merely but the opening of his discoveries. He found out the fecret to fee the vibrations or fits of light, which come and go inceffantly, and which either tranfmit light, or reflect it according to the denfity of the parts they meet with. He has prefum'd to calculate the density of the particles of air neceffary between two glaffes, the one flat, the other convex on one fide, fet one upon the other; in order to operate fuch a tranfmiffion or reflexion, or to form fuch and fuch a colour. FROM FROM all these combinations he difcovers the proportion in which light acts on bodies, and bodies act on light. He faw light fo perfectly, that he has determin'd to what degree of perfection the art of increafing it, and of affifting our eyes by telescopes, can be carried. DES CARTES, from a noble confidence, that was very excufable confidering how ftrongly he was fir'd at the first discoveries he made in an art which he almost first found out; Des Cartes, I fay, hoped to discover in the ftars, by the affistance of telescopes, objects as small as those we difcern upon the earth. BUT Sir Ifaac has fhewn, that dioptric telescopes cannot be brought to a greater perfection; because of that refraction, and of that very refrangibility, which at the fame time that they bring objects nearer to us, fcatter too much the elementary rays; he has calculated in thefe glaffes the proportion of the fcattering of the red and of the blue rays; and proceeding fo far as to demonftrate things which were not fuppos'd even to exift, he examines the inequalities which arife from the shape or figure of the glafs, and that which arifes from the refrangibility. He finds, that the object glass of the telescope being convex on one fide, and flat on the other, in cafe the flat fide be turn'd towards the object, the |