tare of the Prifm without any variation of the length ; Or, if 
the apertte be augmented on all fidesj the image on all fides 
will be fo much and no more augmented, . 
3, Of the //'/W experiment I have cGcafion tofpeak in my 
anfwer to anbther perfon 3 where you i find the efFeds of two 
lirifms in all crofs pofitions of one to another defcribed* But 
if one Prifm alone be turned about, the coloured image 
will only be tranflated from place to place, defcribing a cir- 
cle or forne other ConickSedliononthe wallj on which it is 
projededj without fuffering any alteration in its fl?ape > unlefs 
iuchas may arife from theobliquity of the wall orcafual change 
of the Prifms obliquity to the Suns rays^ 
The effed: ot ih^ fourth experiment I have already infi- 
nuatcd telling you(in pag.3076 of the TranfaHioris^ that Light^ 
paffing through parts of the Prifm of divers thicknelTes , did 
ftiii exhibit the fame Phenomena. 
iVo/(?3that the long axes of tl" e two Prifms in the ekperiment 
defcribed in the faid pag, 3076 of the TranfaBionsl were paraU 
lei one to another^ And for the reft of their pofition^yo'^ will 
btft apprehend it 
by this Scheme ; 
where let EG de- 
fign the window ; 
F the hoI3 in it 5 
through which the 
light arrives at the 
Prifms ABC the 
firjl Prifm 5 which 
refrafts the light 
towards PT,paint» 
ins: there the cos 
lour in an oblong 
form ; and ^i?^ the fecondVrtim, which refracts back again the 
rays to where the Jong image PT Is contracted into a rbuod 
oner 
The plane to BC, and to AC,vI luppofe parallel ^ that 
the rays may be equally refracted contrary ways in both Prifms. 
And the Prifms muft be pkced ^erftie^i^to^ie another For 
