( - 6 ? )

Image (horter than I did, ßeing he tryed the Experiment with a lclsAngle.

The Angle indeed which I uscd was butabout 6; degrees, n min.and hisisfet down 6o degrees: the difierence of which front mine, being buc 3 deg.ii ntin. is too little to reconcile us, but yet it will bring us confiaerable »eur-er together. And if his Angle was not exactly measored,but the round num-ber of 6o degrees ßt down by guesi or by a lefs accurate nteasure sas 1 füf-pect by the conjectural measure of die Refraction of bis Prifm, by the Ratioof the Signs i to 3, fet down at the lanie time, instead of an Experimental-onc,J thcn ntight it be two or three degrees less then 6o deg. if not still lefs :And all this, if it (hould be tb, would take away the greatest part of thedifierence between us.

But however it be, I am well astured, nty own obßrvation was exactenough. For I have repeatcd it divers times since the receipt of Mr. Lu-ens'' s Letter, and that witnout any confiderabledifierence of my Obfervations,either from one another, or from what I wrote bcforc. And that it might ap-pear experimentally, how the increaß of the Angle increaßs the Length ofthe Image, and also that no body, who has a Mind to try the Experimentexactly, might be troubled to procure a Prifm which has an Angle just of theBignefs allignedby me; I tryed the Experiment with divers Angles,and havefet down myTryals in thefollowing Tabie; whcrethefirst Column expreffesthe Angles of two Prüfns which I used, which were measured as exactlyas I could by applying them to the Angle of a Lector ; and the fecond Co-lumn exprefles in inches the Length of the Image made by each of thoß An-gles ; its Brcadtb. being two inches, its Distance from the Prifm 18 Feet andfour inches, and the Breadth of the Hole in the Window-(hüt 4 of an inch.

Angles

Length s

Angles

LcwtL

Cy6A 10'

74

Cf4°- 0'

.

/ ?

The first Prifm60.14

9t

The fecond Prifm P 6i . 11

ioy

£63 . a.6

iof

<463 . 48

!Oi

You may pereeive, that the Length of the Images, in refpect of the An-gles that made them, are lbmething greater in the fecond Prifm than in thehrst; but that was becauß the Olafs, of which the fecond Prifm was made,had the greater Refractive Power.

The Days in which I made theß tryals were pretty Clear, but not so clearas I delired; and therefore afterwards meeting with a Day as Clear as I desired,I repeated the Experiment with the fecond Prifm, and found the Lenvths otthe Image made by its several Angles, to be about 4 of an inch greater thanbefore, the meafüres being thoß fet down in this Table.

The fecond Prifm

Z

Angles

Lengths

74 °

0*

yf

6 z .

11

10 k

63 .

48

ll

The.

Vol.I.