C S6t ) 
4» The Diameter of the hole I put |^ of an inch So^ jf>. 307 7 , 
and placed the Prifm clofe to it, even fo clofe as to be contiguous-* 
M 8o,/>. 3077,//;^. 4, 5. But yet there needs nocuriofity in (hefe 
circumfhnces. The hole imy be of any other bignefs^and the Prifm 
at a diftancc from the hole, , provided things be fo ordered, that 
the light appear of a round form, if intercepted perpendicularly at 
its conning out of the Prifm. Nor needs there any curiofity in the daj. 
The clearer it is the better ; but if it be a little cloudy, that cannot 
much prejudice the Experiment, fo the Sun do but fhine diftinfily 
through the cloud* 
Thefe things being thus ordered, if the refra<aed light fall per- 
pendicularly on a wall or paper at 2ofoot ormore from the Prifm, 
It will appear in an oblongform^crofsto theaxisof xhePrifm^re-i 
at one endjand violet at the other^ the length five times the breadth 
(more or lefb according to the quantity of therefraftion,/ the fides, 
ftreight lines, parallel to one another, and the ends confufed, but 
yet feeming femi- circular. 
I hope therefore^Mr, Lim\ Friends will not entertain them- 
felves any further about incongruous /»m;y^i, but try the Experi- 
n^ient as Mr. Gajcow has promifed. And then, fince Mr. Gafcoi» tells 
you. That the Experiment being of it [elf extraordimry and furpri- 
zing , and bejldes ujhering in new Principles into Ofticks.quite con- 
trary to the common and received , it vpill be hard to perfrvade it a 
truths till it he made fo vijible to alias it were a Jhame to deny it', if he 
efteem it fo extraordinary , he n>av have the priviledg of making 
it fovifibletoall, that it will bea fhame todeny it. For, I dare 
fay, after his teftimony nobody elfe will fcruple it. And I make no 
queftionbuthe wi!l hit of it, it being fo plain and eafy, that I am 
very much at a lofs toimagme w hat way Mr. Linus took tomifs. 
Cambridge Feb. 29. 1 6 j^. 
An ExtraEi of a Latin Letter of Signor CafRni, containing both his Con^ 
Jtderations upon Air. fUm^Qcd's accomt of the Lunar 
Mclipfe o/Decemb.2i. 1675 t» andhisorcn Obfervati^ f See iV» izu 
omof thefameEcliffe. TrTis"^^^^'^^^ 
Ciariffitno Vira 
Dora. Henr. Oldenburg Reg. Soca Sccrctis 
J.DQmimQXXsCaffinm S. P, 
OBfervati^ Lmarii Eclipfii^ m^e pr<z cedent e diem frimam^^Timtn miii 
hnjm celebratte, qmm mhi ado^iffima Plainfteaio cetHmmicafli^inter 
hi^cillimas recenfenda eji.Obliqm qkippe Lfina ineidentia in Vmbr^m^ in hoc 
D d d d 2 f^i V9 
