coiurary refra(ftionpf a fecopd Prifm, further explained io Knm. 
83, />. 406 1 , For thi$ is not to be done w Mr. hitm furmife of a 
parallel Iinage,and therefore had Mr. hims confidered ic^ he could 
never have run into that funnife. 
This I fuppafe is enough tomanifeft the three particulars ; any 
one of which being evidenced,is fufficient to takeaway the fcruplc. 
And therefore lAx.Lims Friends need not fear but that the further 
direii-ions I fent them lately fpr^ry ir^ the Exp^ritBenc are the fame 
with thofe I have follow'd from the beginning \ nor trouble them- 
fel ves about any thing but to try the Experiment right. But yer, 
becaufe Mr. Gafioin has been pleafed to infinuate his fufpicion that 
I do differ from himfelf in thofe direftions, 1 (hall not fcruplc here 
to reduce them into particulars, and fljcw where each particular 
is to befound* -^^ . 
1. Then,heistoget aPrifm with an angle about 60 or 65 degrees, 
M 8o^ f. 3077, and f. 30S6. If the angle beabout 63 degrees, 
as that was which I madeufe of N, 80. f.3077, he will find all 
things fucceed exaftly as I defcribed them there. But if it be bigger 
or kfi, as 30, 40, 5o^Qr 70 degrees, the Refrafiion will be accord- 
ingly bigger or lefs, and confequently the Image longer or Ihorter, 
I,f his Prifm be pretty nearly equilateral (fach as I fuppofe are ufu» 
ally fold in other places^s well as in Engimd ) he may make ufe of 
the biggefl: angle. But be mnft be fure to place the Prifm fo^ that 
theRefraftionbemade by the two planes which comprehend this 
angle* I could almoft fufpeft , by confidering fome circumftances 
in Mr. Linm\ Letter, that his error was in this point, he expefling 
the Image iliOuld become as long by a little refraflion as by a great 
ope ; which yet being too grofsari error to be fufpefted of any O- 
ptician, I fay nothingofit., b^tp^^lyhinttb^isto Mr* that 
he may examine all things* 
2. Having fuch a Prifm, he muft place it fo, that its Axis be; per- 
pendicular to the rays 84,/». 4091,///;. 18, 19. A littJe. error 
in this point n^iajies no feqfible variation of theeffefl:. 
3. The Prifin muft be fo placed, that the Refraftions on both fides 
be equal 80, p. 3077 ; vyhich how it was to be readily done by 
turning it aboutiits Axis, and flaying it when you fee the Image 
reft between too contrary motions, as I e5^plained in my late De- 
fcriptions,fo I hinted b^iQYeN.ic.p.^o-jjJin, 34,35, 36^ Ifthere 
fliould be a little error i^^ this point alfoi it can do no hurt. 
4. The 
