( 6i o ) 
both appearing fringed with Colours when feet* 
through the Prifm. Then holding two Prifms to- 
gether, as in Fig. io, if the Hole of the dark Cham- 
ber be at k, it appears white to the Eye at E ; bat 
if the Angles of the Prifms at B F be a little fepa- 
rated, whiift the Points A D touch, the Hole will 
appear coloured : When the Surfaces are feparated 
at A D 5 and touch at B F, the Colours appear in an 
inverted Order ; but if the Surfaces A D and B F are 
parallel, whether they touch or not, the Hole will 
appear white. 
N. B. In this Cafe the 'Prifms muft be fimilar^ 
that the Surface F G may be parallel to AC ; other - 
wife A B and D F muft be fo inclined to one another 
as to render A C and F G parallel. Indeed if one 
of the Priftns be very far removed from the other , 
the heterogeneous Light which entered in at F G, 
may be fo far fpread by the Separation of the dif- 
ferently refrangible Rays , that the Prifm ABC 
will not take it all in , then the Eye behind the fe - 
cond Prifm may fee Colours , as I fuppofe Riz- 
zeti did. See Page 79 of his Book. 
If the Ray of Light R a b c d d ( Fig. 15. ) 
changing its Diredion in the Manner above-men- 
tioned, makes an Angle of about 45 Degres with 
the Perpendicular P a , upon the removal of the 
lower Prifm, the Ray will be turned up again, as 
in Fig. 14. But if the Angle P^R be greater, the 
Ray will Bill be turned up again in a Curve, as 
a b c d e ft ( Fig. 16 ) notwithftanding the lower 
Prifm is at D F G ; but if that Prifm be brought up 
clofer to the Surface A B, the Curves will be destroyed 
where the Prifms touch, and all the Rays in the Place 
of Contad brought down through the lower Prifm. 
THE 
