exhibited by thin plates. 
165 
and were placed in the same order with respect to each other ; 
that is, beginning with red, next orange, then yellow, green, 
blue, indigo, and lastly, violet : but that the fringes them- 
selves were divided into two classes, (with respect to the 
position of the colours of each particular fringe) by a central 
band or fringe passing through a point bisecting the distances 
between the centres of the primary set and its image, each 
fringe, on both sides of this imaginary point, respecting it as 
a centre, and having the red colour of each fringe turned out- 
wards, or from that centre; the central band alone being equal 
on both of its sides with respect to colour ; being as it were 
composed of two of the inside halves of the two adjoining 
fringes, imagined to be laid together ; having no red in its 
composition, and being rather less intense in colour than any 
of the others. An imitation is given in fig. 1. pi. VII. and as it 
would be impracticable to express all the colours of each fringe 
by shadowing, the darkest shade denotes the violet, the un- 
shaded side the red of each fringe, and the black lines the 
divisions between them. 
Exp. 4. Another mode of producing those fringes was by 
applying a convex and a concave lens together, the curvature of 
the one differing but little from that of the other. This appa- 
ratus produced a larger set of primary rings, and consequently 
broader fringes, than by either of the foregoing experiments ; 
but in this case none of them were straight, except the cen- 
tral fringe, all the rest being bent more or less into elliptical 
shapes, conformable to the surfaces between which they were 
formed. 
Exp. 5. But a better mode still was found, by applying a 
slip of looking-glass plate to one of the sides of a triangular 
