174 Mr. Knox on some phenomena of colours, 
formed by the two sets of primaries, that angle was always 
bisected by the central band of the intersectionaries. And 
although these latter evidently proceed from the crossings of 
the two sets of primaries, yet they are never continued through 
those crossings to the opposite angle at A, hg. 6, (pi. VIII.) 
nor could they be made to appear in any angle formed by 
primary fringes, unless the said fringes were so disposed as to 
have their red sides turned inwards, or towards each other. 
This remarkable fact was proved by several repetitions of the 
same experiment. 
Exp. 15. In the course of making these experiments, I had 
accidentally left a single slip of glass on one of the pairs be- 
tween which a set of primaries were formed, without any 
other pressure than its own weight ; on examining the appara- 
tus a few minutes afterwards, I was agreeably surprised to 
find no less than four sets of the same kind of fringes which 
appeared in the angle between the two sets of primaries, as 
before related in Experiment 12, almost parallel to each other, 
and nearly so to the primaries, and at about an inch distant 
in front thereof, (see fig. 5. pi. VIII.) The set marked 2, 
had broader fringes, more vivid in colour, and consisted of a 
greater number, (from fifteen to nineteen), than those sets 
on each side thereof ; for these latter appeared to have that 
inferiority of colour to the principal set, that it bore to the 
primary set. All the four sets were by attentive observation 
visible to the naked eye, and were all divided into two classes 
by a central band, as was before observed of the others ; and 
on application of the shadow, were found to consist of pri- 
mary and transmitted fringes, precisely in the same manner 
as first primaries, or Newtonian rings. 
