( 444 ) 
pafs thro’ the Hole h, arid fall obliquely upon the fecond 
Prifm B. This Ray after its Refra&ion in patting thro* 
the fecond Prifm,was carried up to theCeilingof the Room 
at the place markd R: then I made the purple Ray fall 
upon the Board, and pafs thro’ the Hole h, as the Red had 
done before; and after Refra&ion thro* the Prifm B it was 
carried up to the Ceiling at P. And the green Ray being 
afterwards made to pall the fecond Prilm in the fame 
manner, went up to 6 : and fo of all the intermediate 
Rays, which were by this fecond Refra&ion thrown to the 
intermediate places on the Ceiling between/? and P. 
Care is to be taken that the fecond Prifm be plac’d 
oblique to the Rays which come thro’ the Hole £,leaft they 
be reflected, as they wou’d be, if the Board being in the 
Pofition JUjS, and the fecond Prifm in the Pofition LNM , 
the Ray from the firft Prifm be y h for then it will be re? 
fle&ed upwards to o-inltead of being refra&ed ( Fig. 19.X 
Neither mull the Plane of Immerfion be too oblique, leaft 
the Incident Ray be reflected downwards by it, as the Ray 
R h is by the Prifm 3 thrown to E, in Fig, 10. Several have 
confefs’d tome that they at firft: us’d to fail in this Expe- 
riment, for want of Petting the fecond Prifm in a due In- 
clination 
Tho* the Colours by the lecond Refra&ion on the Cei- 
ling appear’d unchang’d, when leen by the naked Eye, yet 
tf view’d thro’ a Prifm, they afforded new Colours (ex- 
cept fome part, of the Red, and- Come part of the Violet j 
which was owing to their not being fully feparated ; for 
which reafon 1 made the following Experiment, to prove 
that if the Colours be well feparated, they are truly ho? 
mogeneal and unchangeable. 
N. B, Vf hen the Prifms are good,and no Clouds are near 
the Sun, theExrremity of the Red or Violet will afford 
u&mix’d Colours in this Experiment ; otherwile nor, 
