the Cause of Coloured Concentric Rings. 151 
Newtonian experiment will be considered in the following 
articles. 
LIV. Supplemental Considerations , which prove that there are two 
primary prismatic Bows, a blue one and a red one. 
As it will be admitted that we have a primary blue bow, I 
shall only repeat that by the use of the criterion, which has 
been indicated at the conclusion of my 44th article, we find 
that when a plain surface of glass is brought into contact with 
that side of the prism by which the reflective, or intromissive 
critical separation is performed, the bow will be turned into 
streaks, and that the blue bow, which Newton has explained, 
will stand the test of this criterion. 
It will now be necessary to prove that the red bow which I 
have introduced in my 43d article is a phenomenon of equal 
originality with the Newtonian blue bow. That it will stand 
the test of the criterion, has already been proved in the 44th 
article, since by the contact of a plain surface of glass with 
the efficient surface of the prism this bow is also turned into 
streaks ; but as we find that the Newtonian experiment, by 
the addition of a second prism, has made the red, orange, and 
yellow colours, which are the residue of the blue bow, visible, 
it will be necessary to show that the phenomenon which may 
thus be viewed is not the red bow I have described. 
First consideration. It is a necessary consequence from 
Newton’s explanation of the 49th figure in his Lectiones 
Opticce, page 260, for a copy of which see Plate V. fig. 1 ; that 
the angle t s p subtended by the transmitted colours must 
be exactly equal to the angular breadth of the blue bow t S p 
and that also the angular position Ct s, when the eye sees 
X 2 
