( 504 ) 
perpendicular to the Sun-beanis,and then turn it about its axu^ihey 
cannot niifs of feting the Jirjl Image; which having found,ibey u)ay 
double up a paper once or twice, and make a round hole in ihe 
middle of it about ^ or ^of an inch broad, and hold the paper im- 
mediately before the Prifni, that the Sun may (hine on the Prifm 
through that hole ; and the Prifm being ftay*d, and held fieddy in 
that pofture which makes the Image Stationary ; if the Image then 
fall diredlly on an oppofite wall, or on a flieet of paper placed at 
the wall, fuppofe 1 5 or 20 foot from the Prifiii, or further ofFjthey 
will fee the Image in fuch an Ol^Ung figure as I have defcribed,with 
the iJ^^ at one end, the r/^>/e^ at theoiher, and a Elervijb gree?^ h\ 
the middle: And if they obfcure their Room, as much as they can, 
by drawing curtains or otherwife, it will make the Colours the 
more confpicuous. 
This direftion I have fet down, that no body, into wbofe hands 
aPrifm Cball happen,may find difficulty or trouble in trying it.Buc 
when Mr.L/^^i's friends have tryed it thus, they may proceed to 
repeat it in a dark Room with a lefs hole made in theirwindow fliur. 
And then Ilhall defire,that they will fend you a full and clear de- 
fcription,How they tryedit, cxpreffing the length, breadch and 
angles of the Prifm; its poficion to the Incident rays and to the 
window (hut ; the bignefs of the hole in the window (hue through 
which the Sun ftined on the Prifm ; what fide of the Prifm the Sun 
ihin'd on ; and at what fide the light came out of it again ; the di- 
fiance of the Prifm from the oppofite paper or wall on which the 
Refrafted light was caft perpendicularly ; and the length,breadth, 
and figure of the fpace there illuminated by that light,and the fci- 
tuationof eachcolour within that figure* And, it they pleafe to 
iliuftrate their defcription with a Scheme or two, it will make the 
bufinefs plainer. By this means, if [here be any difference in our 
way of experimenting, I fiiall be the better enabled to difcern ir, 
and give them notice, where the failure is, and how to reftifie it. I 
iliould be glad too, if they would favour me with a defcription of 
the Experimenr,as it hath been hitherto tryed by Mr.Lhus, that I 
may have an opportunity to confider, what there is in that which 
makes againft me. 
St^/^r Mr. Newton; which was thought fit to make publick with 
the refl-,that fo the Curious every where, who have a mind to try 
the Experiment, may find the fuller direflions for their tryal 
