( 44? ) 
at JTwill fee this Eeam at Toblongand colour’d, and the 
Letters on which it falls, confus’d. SeeJ%. 2.4. 
N. B. The Lens ought to be very good, without 
Veins or Blebs, and ground to no lets a Radius than l 
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* Giafles of Telefcopes, the white 
Glafs, of which frifms are ufually made, being com- 
monly full of Veins And the Room in thefelaft 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 Glals, 1 made all 
the Experiments over again before feveral Members of 
the Royal Society with betcer Succefs ; and had the Speffrum 
very regularly terminated, without any pale Light dart- 
ing from the Ends of it- 
For a further Account oj Experiments to this purpofe , fee 
Sir Ifaac Newtonb Opticks. B. 1 . Part, r . to which l might 
have referr’d the Reader altogether; but that 1 was wil- 
ling to be particular in mentioning fuch things as ought to 
be avoided in making the Experiments above* mentiorfd ; 
fome Gentlemen abroad having complained that they 
had not found the Experiments anfwer, for want of fuffici- 
ent Directions in Sir Ifaac Nevctons Opticks ; tho’ I had 
no other Directions than what I found there, 
