C 616 y 
(i the Part of the Bafe I M N G beyond this Bow will 
“ be brighter than the other Part EMNH on the o- 
“ ther Side of it. Now for underftanding the Reafon 
“ of it, fuppofe the Plane ABC to cut the plane Sides 
<c and the Bafe of the Prifm perpendicularly. From 
<s the Eye to the Line B C wherein that Plane cuts the 
“ Bafe, draw the Lines S p and St in the Angles S/C 
u 50 Degrees ~ a °d S t C 49 Degrees *, and the 
“ Point p will be the Limit beyond which none of 
u the mod refrangible Rays can pafs through the 
6i Bafe of the Prifm,. and be refraCted, whole Inci- 
“ dence is fuch that they may be reflected to the 
“ Eye ^ and the Point t will be the like Limit for 
“ the lead: refrangible Rays, that is, beyond which 
l£ none of them can pafs through the Bafe, whofe In- 
u cidence is fuch, that by Reflection they may come 
“ to the Eye. And the Point r taken in the middle 
“ Way between p and will be the like Limit for 
u the meanly refrangible Rays. And therefore all 
“ the lead refrangible Rays which fall upon the Bafe 
« beyond t } that is, between t and B, and can come 
u from thence to the Eye will be reflected thither : 
u But on this Side t> that is, between t and C, many 
tc of thefe Rays will be tranfmitted through the Bafe: 
* And all the mod refrangible Rays which fall upon 
tc the Bafe beyond /, that is, between p and B and can 
w by Reflection come from thence to the Eye, will be 
“ reflected thither, but every where between p and r, 
u many of thefe Rays will get thro 9 the Bale and be 
u refraCted ^ and the fame is to be underflood of the 
*■ meanly refrangible Rays on either Side of the Point 
u r. Whence it follows, that the Bafe of the Prifm 
mud every where between t and B by a total Re* 
£ fleCtion 
