placed dose to the Hole but in a competent dillance front the sime, thenthesc Semicircular Ends always appear with the Sides thereof streight Lines,just as Mr. Newton describes them. Neither is the Lengthof the Image Trans-verse, but Parallel to the Length of the Prism Out of all which evidenti/follows, that the Experiment was not made in a ciear day; nor with thePrism dose to the Hole; nor yet with the Image Transverse, but by a brightCloud and a Parallel Image (as I conjectured ;) and I hope you will alio nowsay, I bad good Rcason sb to Conjecture, since it so well agrees with the Rela-tion. And Experiencc will also sltew you, is you pleasc to malte Tryal, asit was made in a Dark Chamber, and oblerve the difterence between such an

Image made by a bright Cloud, and another made by the Immediate Raysof the Sun: For, the former you Ihall always find Parallel, with the Ends Se-micircular; but the lattcr you Ihall find Transverse, with the Ends Pyranti-dical, as aforesaid, whensbever it appears so much Longer than Broad.

Morc might be säid out of the säme Relation, to shew that the Image was notTransverte. For if it had been Transverse, Mr. Newton, so well skill’d in Optickj,■could not have been furprilcd (as he läys he was- to lee the Length thereof somuch to exceed the Breadth; it being a thing so obvious and easie to be Explica-ted by the Ordinary Rules of Refraction. That other Place also (where he säys,the Incident Refractions were made in the Experiment cqual to the Emergent ,)proves again that the said Oblong Image was not Transverse, but Parallel. Forit is impolllble, the Tranlverse Image should be so much Longer than Broad,unless those two Refractions be made very Unequal, as both the Computationaccoidin" to the comrnon Rules of Refraction, and Experience tellify.

4. What it is that impoles upon Mr. Line 1 cannot imagine ; but I saf-pect he ltas not tryed the Experiment since he aequainted himself with myTheory but depends upon bis Old Notions, taken up before he had any hint

iAnsrver',1 by As»'#

Ke\vton.n.i2i«

to observe the

Figure of

the Coloured Image. I Ihall desire him

501. n. 123.;; 6 .

theresore, before he returns any Answer, to try it once more for bis Satis-faction, and that according to this manner.

Lct him take any Prism, and hold it so that its Axis may be Perpendicu-lar to the Sun's Rays, and in this Posture let it be placed as dose as maybe to the Hole through which the Sun Ihines into a dark Room, whichHole may be about the Bigness of a Pease. Then let him turn the Prismflowly about its Axis, and he Ihall see the Colours move upon the oppo-site Wall, first towards that place to which the Sun’s Direct Light would pass,if the Prism were taken away, and then back again. When they are in themiddle of thele two contrary Motions, that is, when they are ncaresl: thatplace to which the Sun’s Direct Ray tends, therc let him stop ; for then are tl eRays equally Refracted on both Sides the Prism. In this Posture of the Prismlet him oblerve the Figure of the Colours, and he Ihall find it not Round, ashe contends, but Oblong, and so much the more Oblong as the Angle of thePrism, comprehended bythe Refracting Plains, is Bigger, and the Wall, onwhich the Colours are cast, more distant front the Prism; the Colours Red,Yellow, Green, Blue, Purple, lucceeding in order not front one Side of tlieFigure to the other, as in Mr. Lines conjecture, but front one End to the

Y 1 other;

\