( 40^1 ) 
ture of the Prifm without any variation of the length : Or, if 
the aperture be augmented on all fides, the image on all fides 
will be fo much and no more augmented, 
3, Of the third experiment I have occafion tofpeak in my 
anfwer to another perfon ^ where youl find the effedts of two 
Frifms in ail crofs pofitions of one to another defcribed* But 
if one Prifm alone be turned about, the coloured image 
will only be tranflited from place to place , defcribing a cir- 
cle or fome other ConickSedliononthe wallj on which it is 
projeded, without fuffering any alteration in its (bape ^ unlefs 
fuch as may arife from the obliquity of the wall orcafual change 
ofthe Prifms obliquity to the Suns rays* 
4» The effedl ot the fourth experiment I have already infi- 
nuated telling you(in pag,3076 ofthe 7r^Y^ffi^/?/) that Lighr^ 
paflSng through parts of the Prifm of divers thickneffes , did 
ftill exhibit the fame Phaenomena. 
. 2Vo/^,that the long axes of tl e two Prifms in the experiment 
defcribed in the faid pag* 3076 of the TranfaBions^ were paral* 
lei one to another* And for the reft of their pofitioUjyo^ will 
bcft apprehend it 
by this Scheme ; 
where let EG de- 
fign the window ; 
F the hole in it, 
through which the 
light arrives at the 
Piifms^ ABC the 
/r/? Prifm, which 
refrafts the light 
towards PXpaint* 
ing there the cos 
lour in an oblong 
form ; and etfy the fecond Prifm, which refracts back again the 
rays to where the long image PT \% contracted into a round 
one. 
The plane<t>. to BC, and fiy to AC, I fuppofe parallel , that 
the rays may be equally refracted CGntrary ways in both Prifms. 
And the Prifms muft be placed very near to one another • For 
' if 
