(.*!? ) 
There are two Circumftances in the 6th Ex- 
periment, which difprove Rizzetti ' s AiTertion 
(Page 1Z5) viz. That there is a feniible Reflection even 
where Glades touchy for when the Prifins touch at I 
Fig. 10, the black Spot appearing in the Image of the 
Candle k fhews that there is at I a Deficiency of thofe 
Rays, which, coming from the Middle of the Candle, 
ufed to be receded up to the Eye at E, and therefore 
that A B the refieding Surface of the upper Prifm 
ceafes to refled in a little Space round about I where 
the upper Surface D F of the under Prifm touches it j 
the Rays, which before were refleded, now going down 
to make the Spot at R. The other Circumftance is 
this ; that whereas a Paper at k is invifible to an Eye 
at E by the Interpofition of the Prifm DFGj when 
another Prifm A C B is laid over it and preffed hard, 
there appears to be an Hole of about ± of an Inch (more 
or lefs in Diameter as the prifmatical Surfaces are more 
or lefs flat) thro’ which the Paper at k becomes vifible > 
this being the Place of Contad where the Refledion 
downwards (of the Surface D F) ceafes. 
This happens becaufe thofe Rays, which (coming, 
from the Candle K) were bent in Curves under the Sur- 
face A B of the upper Prifm about feveral Points near 
I, are by the Nearneft of the Surface D F of the lower 
Prifm brought down to R, inftead of being turned up 
again to the Eye at E; whilft thofe Rays, which (com- 
ing from the Paper at k thro’ the Surface GF of the 
lower Prifm, and palling thro’ the upper Surface of it 
FD) were bent in Curves about feveral Points near I, 
are prevented from turning down again to R, and are 
