An Account of an Optical Experiment Made before the Royal Society, on Thursday, Dec. 6th, and Repeated on the 13th, 1722. By the Reverend J. T. Desaguliers, L L. D. F. R. S.

Crystallum istam in complura frusta confregi, & frusta microscopiis applicavi; disquirere volens num & illa ex superstratis sibi lamellis composita essent: quâ ratione Adamantes magnitudinem fuam adepts esse dixi. Sed, tametsi perquisitionem istam fæpius iteraverim, ne tantillam quidem lamellam in iis deprehendi. Istud autem in Crystallis, quas quidem præmanibus habebam, plerumque animadverti, in omnibus earum lateribus, quæ numero sena erant, transversas protendi lineolas, alias aliis situ aliquantulum superiores; tamquam si illic, increcentibus Crystallis, ortæ productæque fuissent: quâ super re, quantumcumque Crystallorum numerum ante consideraverim, atque confregerim, numquam ipse mihi satisfacere potui. Delphis in Hollandia, Nov. 20. 1722. IV. An Account of an Optical Experiment made before the Royal Society, on Thursday, Dec. 6th, and repeated on the 13th, 1722. By the Reverend J. T. Desaguliers, L.L.D. F.R.S. Sir Isaac Newton, in his Optics, (B. I. Prop. I. Exp. 2.) relates an Experiment made with a Card, or Paper, painted red on one half and blue on the other, which being enlighten'd by a Candle, the Image, by the Interposition of a Lens, is so projected on a white Paper, held on the other side of the Lens, that the Place where the blue half appears distinct, (or as the Opticians term it, the distinct Base of the Image of the blue half) is much nearer to the Lens than the Place of the Image of the red half. And this is made apparent apparent, by seeing on one of these Images the Representation of the black Threads, that are wrapp'd round the Card, whilst they are not visible on the other. This is fully described in the Place abovemention'd; but yet a * Gentleman abroad has call'd the Experiment in question, and denied the matter of Fact, saying, that he could not make it succeed, but proposes an Experiment of his own to disprove the different Refrangibility of the Rays. Upon this I was desir'd to make the Experiment over again, before the Royal Society, which succeeded well. But because there must be care taken in the making it, I shall mention all the Particulars observed in the Performance; which, if duly put in practice, will make the Experiment always succeed. I painted one half of the Card RB, (Fig. X.) as B, with Ultramarine, made deeper with a small Mixture of Indigo, and the other half R, I painted over with Cinnabar heighten'd with a little Carmine, so that the Line, that separated the red from the blue, was perpendicular to the long sides of the Cord. Then I wrapp'd a black Silk four times together, over the middle of each painted part of the Card, as in Fig. XI. Upon a square Trencher, Fig. XII. painted black, and suspended vertically against a Wall, I fix'd my colour'd Card with a Pin, and the Room being made very dark, I enlighten'd the Card with a strong Light thrown upon it from a dark Lanthorn, that had two Convex Glasses in it; then setting up the Lens LL, (represented by Fig. XIII.) in such manner, that its Axis pass'd perpendicularly thro' the Image of the Card, at * Act. Erudit. Lipf. Supplem. tom. 8. §. 3. p. 130. 131. the distance of nine Feet from the Card, the Image of the Card being receiv'd upon a white Paper, at the distance of nine Feet on the other side of the Lens, at B, the blue half appear'd distinct, with the Image of the black Silk going vertically along its Plain, whilst no Appearance of the black Silk was perceivable on the red half. Then removing the Paper about two Inches, to R, the red half of the Image had a black Line very plain upon it, whilst it was invisible on the blue half. This was more evident, when a strong I- mage of the Candle was successively thrown on that half of the Card, whose Image was under Examination. When the Paper was held in the middle between R and B, the black Line upon each Colour was visible, but indistinct. N. B. Care must be taken that the Colours be deep, because having accidentally rubb'd off some of the blue, the Whiteness of the Card under it, made its Image fly out farther, almost as far as that of the red.