( ' 7 3 )

ed, is, next to the Experimentum Crucis, the most conspicuous Experiment, .

I know, for proving the Different Refrangibility of Light, which he brings itto prove againff.

By the Post-script of Mr. Lucas s Letter, one not acquainted with what haspaffed, might think, that he quotes the Obscrvation of the Royal Society againffme ; whereas the Relation of their Obfervation, which you sent to Liege, con-tained nothing at all about thejust proportion of the Length of the Imagc toits Breadth aecording to the Angle of the Prism, nor any thing more (so faras I can perceive by your last) than what was pertinent to the things then-in dispute, vi\. that they found them Succeed as I hadaffirmed And there-fore, fince Mr. Lucas has found the tarne Succeß, I soppole, that when he tt-preffed, That he mucb rcjoyced tosec the Tryals cf the R. Society agree fo cxacllywith his, he meant only so far as his agreed with mine.

P. S; I had liky to have forgotten to advife, that the Experimentum Crucis,and such others as fhall bc made for knowing the Nature of Colours , bc madewith Prifms which Rcfra'ci so mucb, as to makc the Lengtb of the Image fivetimes its Breadth, and rather more than less ; for othcrwife Experiments wilt-not succeed so plainly with others as they have donc with nie.

inO Er. VIII. I took a stiff piece of Brown Paper» and pricking.a final 1 Hole-therc-in, I held it at a little Distance before me : then applying aNeedleno my Eye,Steph. Gray. I was surpriled to see the Point of it Inverted. The nearer the Needle wasii, 221.28*. tQ t jj e p^ole, it was lomuch the more Magnifyed, but less Distinct; and if itwere so held, as that its Image was near to the edge of the Hole, its point;l’cemed Crooked. So that, it seems thefe fmall Holes, or somewhat in them,perform the Effects of a Concave Speculum, and so I take leave to call them-Aerial Spcculums.

. IX. i. Mkto tibi hac occasione Constructionenr Problematis Alha^cni nuper

KXhlzenJoivei a me inventam, & a Collegis meis felicem fatis judicatam. Problema est •by M. Chr. Dato Speculo Cavo aut Convexo, itemtpue Oculo c3 PunElo I{ci vise, invenire-n ' 97 • PunBum Reflexionis.

Esto Speculum ex Sphaera quae Centrum habeat A Punctum» Oculus verokso 78' sit in B, & Punctum Visibile in C, Planumque ductum per A, B, C, faciat-in Sphaera Circulum D d, in quo invenienda sint Reflexionis Puncta. Pertria Puncta A, B,C, describatur Circuli Circumferentia, cujus sit Cenrrum Z Toccdrrat autem ei producta A E, Perpend. BC, in R, & sit duabus R A, O A,tertia Proportionalis N A, eritque N M, Paralle ! aBC, altera AlymptotorvRursus sint Proportionales EA, f AO, AI, ScSumma I Y aequali 1 N, duca-tur Y M Parallela A Z ; eaque erit altera Asymptotos. Denique sumptis I X»IS,quae singulae postint dimidium Quadratum AO, una cum Quadrato AI jerunt Puncta * & s in Hyperbola, aut Sectionibus oppositis D d, ad inventasAsymptotos describendis, quarum Interlectiones cum Circumferentia D O,ostendent Puncta Reflexionis quaesita. Constructio haec, in omni Casu, quo--Problema solidum est, locum habet, praeterquam in uno, ubi non Hyperbola

sed