( 44 8 ) 
III. A plain and eafy Experiment to confirm Sir Ifaac 
Newton's Votirine of the different efrangibility 
of the Q^ays of Light. By the fame. 
A Fter the Experiment urn Cruets made by two Prifms, 
1 fhou’d not give the following Experiment, but 
that it is fo eafy to be made, that by it thofe who want 
the Apparatus (or are unwilling to be at the pains) to 
make the Experiment um Qrucis , may at any time fatisfy 
themfelves of the Truth of the fore-mention’d Do&rine. 
Let the Candle A be fet before the Bar of a Chimney 
Looking* Glafs, fuch as is reprefentedby/7//(7 r /g-. a 5 ) 
which is a Piece of Looking-Glafs Plate confifting of four 
Planes, feenin the Section of it <zfd(2, viz. d (3 which 
is quick-filver’d behind, fee a Plane parallel to it, fd one 
of the Side-planes bezell’d towards d (3, or inclin’d to it 
in an Angle of about 40 Degrees (tho* from 30 to 40 will 
do, but the greater the Angle the better, if it does not 
exceed 45 °..) a (3 the other Side- Plane inclin’d in the fame 
Angle to (3 d. 
The Rays of the Candle which come from A toy fall 
obliquely on the Plan ea/6, fo that inftead of going on to 
a, they are by Refra&ion made to incline more towards the 
Perpendicular p p, namely to go on in the Line y e, and 
then are reflected from the Point c on the quick-filver’d 
Surface, in the Dire<3ion fx, fo as to make the Angle 
*tc d ~y c (3. Now as the Rays which wou’d go to x, 
if not refra&ed, emerge obliquely from the Plane a j3, they 
leave the Direction ex, and decline from the Perpendicu- 
lar 7 r 7r, and, being differently refra<5ted, open into four dif- 
ferently colour’d Rays ; viz . b R a red Ray, b TO a Ray 
made 
