( 44? ) 
at LTwill fee this Beam at Toblongand colour’d, and the 
Letters on which it falls, confus’d. See Fig, 2.4. 
NIB. The Lens ought to be very good, without 
Veins or Blebs, and ground to no leis a Radius than 1 
mention’d in the Experiment ; tho’ a Radius of a Foot or 
two longer is not amifs. The Prifm ought to be of the 
fame Glafs as the Objeft-G iafles of Telefcopes, the white 
Glafs, of which frifms are ufually made, being com- 
monly full of Veins And the Room in thelelalt Experi- 
ments ought to be very dark. 
A few Days after, having got very good Prifms made 
for the purpofe of the above mention’d Gials, 1 made all 
the Experiments over again before feveral Members of 
the Royal Society with better Succefs ; and had the Speffrum 
very regularly terminated, without any pale Light dart- 
ing from the Ends of it- 
For a further Account of Experiments to this purpofe, fee 
Sir ifaac Newtonb Opticks. B. 1. Part, t . to which l might 
have referr’d the Reader altogether ; but that I was wil- 
ling to be particular in mentioning fuch things as ought to 
be avoided in making the Experiments above-mentiou’d ; 
fome Gentlemen abroad having complained that they 
had not found the Experiments anfwer, for want of fufhci- 
ent Directions in Sir Ifaac Newtons Opt id’s ; tho’ I had 
no other Directions than what I found there, 
. . > ■■ — 
L ■ ■ . : tC'* 
