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tVie Image; ’and I measured tfae urmost Length, becausc I accourtt all thatLength to be cauled by the immediate Light of the Sun, leeing the Colours(as I noted above) becomeVisible to the gratest Length in the Clearest Days,that is, when the Light of the Sun tranlcends most the Light of the Clouds.Sometimes there will happen to shoot out from both Ends of the Image aglaring Light a good way beyond thele Colours, but this is not to be rcgar-ded, as not appertaining to the Image. If the Mealüres be taken right, thewhole Length will exceed the Length of the streight lides by about theBi eadth of the Image.

By thele things set down thus circumstantiälly, I prefüme Mr. Luca< willbe enabled to accord his Tryals of the Experiment with mine; so nearly atleast that there Ihall not rentain any very considcrable difterence betwecn us.For, if Ibme littlc difterence Ihould still rentain, that need not trouble usany further, leeing there may be many various Circumstances which mayconduce to it ; such as are not only the different Figures of Prifms, but alsothe disterent Refractive Power ofGlastes, the disterent Diameters of the Sunat divers Times of the Year, and the little Errors that may happen inMeastiring Lines and-Angles, or inplacing thePrisinat the Window ; thoughfor my part, I took Care to do thele Things as cxactly as I could. Howe-ver Mr. Lucas may make füre to sind the Image as Long or Longer than Ihave set down, if he take a Prilm whole Sides are not hollow ground, butplain, or (which is better; a very little Convex, and whole Reffacting An-gle is as much greater than that I used, as that he hath hitherto tryed it withis lels ; that is, whole Angle is about 66 or 67 degrees, or (if he will) a littlegreater.

Concerning Mr. Lucas's other Experiments, I am much obliged to him thathe would take thele things 1b far into Consideration, and be at so muchPains for Examining them; and I thank him so much the more, because heis the first that hath sent me an Experimental Examination of them. But yetit will conduce to his more Ipeedy and füll Satisfaction, if he a little changethe Method which he has propounded, and instead of a Multitude of thingstry only the Experimentum Crucis. For it is notnumber of Experiments 'butweight to be regarded; and where onc will do, what need many ?

The main thing he goes about to examine is, the Different R cfr/mgibUityof Light ; and this I demonstrated by the Experimentum Crucis. Now ifthisDemonstration be good, there needs no further Examination of the thing; ifnot good, the fault of it is to be (hewn: For the only way to examine a De-monstrated Proposition is, to examine the Demonstration. Let that Experi-ment therefore be examined. in the first Place, and that which it proves beacknowledged, and then.if Mr. Lucas want my Allistance, to unfold the disticul-ties which he sanftes to be in the Experiments he has propounded, he Ihall freelyhave it. At prelent I Ihall läy nothingin Anlwer to his Exjrerimental Dilcourle,but this in general, tliat it has proceeded partly from some misonderstanding c fwltat he writes against, and partly from want of duecaution in trying Experi-ments ; and that amongst bis - Experiments there is onc, which when duly try-

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