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held steddy in that Posture which makes the Image Stationary; if the Imagetsiien fall directly on an opposite Wall, or on a Sheet of Paper placed at theWall, soppose i y or 2.0 Foot from the Prifm, or further off; they will fecthe Image in such an Oblong Figure as I have described, with the Red at one.end, the Violet at the other, and a Blucilh Green in the middle: Andif they Obscure their Room, as much as they can, by drawing Curtains orotherwisc, it will make the Colours the more confpicuous.

This Direction I have sct down, that no Body, into whosc Handsa Prifm(hall happen, may find difficulty or trouble in trying it. But when Mr. Linus’sFriends have tryed it thus, they may proceed to repeat it in a dark Room-with a lefs Hole made in their Window-lhut. And then I lhall desire thatthey will scnd a füll and clear description how they tryed it. I Ihould beglad too, if they will favour me with a description of the Experiment as itbath been hitherto tryed by Mr. Linus, that I may have an Opportunity to con-sidet what therc is in that which makes against me.

7. Mr. Gafcoigne wanting Gonvenience to make the Experiment, according Exeepiiom ly.•to the frelh Directions from Mr. Newton, requested me to fupply his Want. Mr - LlK1 *-

The Vertical Angle of my Prifm was 60 Degrees; the Distance of the I28 ’P Ä?a 'Wall, whereon the Coloured Spctlrum appeared, from the Window, about18 Foot; the Diameter of the Hole in the IVindow-Ihuts about £ inch, whichupon Occasions I contracted to half the seid Diameter; but still with equal Success -as to the main of the Experiment. The Refractions on both sides the Prifmwere, as near as I could make thern, Equal, and conscquently about 48 deg.

40min. the Reffactive Power of Glafs being computed. according todte Ra-tio of the Sines ito 3. The. distance of the Prifm from the Hole in theShuts was about x Inches ; the Room darkened to that Degree as to Equalthe DarkestNight, while the Hole in the Shuts was covered.

Now as to the Iffue of myTryals; I constantly found the Length of :the Coloured Image, (Transverse tothe Axis of the Prifm) considerably grea-ter than its Breadth, as often as the Experiment was made on a Clear Däy; .but if a Bright Cloud were near the. Sun, I found- it fometimes exactly asMr. Line wrote you, namely Broader than Long, especially while the Prifm-was placed at a great distance from the Hole. Which Experiment' wilPnor, I conceive, be question’d by Mr. Newton , it- being so agreeable to. the<reeeived Laws of Refractions. And indeed the Obfervations of these two >

Learned Persons, as to this particular, are easily reconcileable to each other,and both to Truth; Mr. Newton contending only for the Length of the Image:

(Transverse to the Axis of the Prifm) in a very Clear day j.. whereas-Mr. Line only maintained the exceß of Breadth,. Parallel to the fame Axis,.while the Sun is in a Bright Cloud. Though as to what is further delivcred-by Mr. Newton , and oppofed by Mr. Line , namely that the Length of the Colou-red Image was five times the Diameter of its BreadthI never yet have found»the excefs above thrice the Diameter, or at most 3 i, while the Refractions.-on both Sides the Prifm were equaL So much as to the matter o£ Fact,

Now as to Mr. Newtons Tbeory of Light and Colours, . I confeß his neac-.

Sett of very Ingenieur and Natural Inferences, was tome upoa the Erst Per*-