on the Affections and Properties of Light. 363 
“ plates.” Happening to look by candle-light upon a round 
concave plate of brass, pretty well polished, so as to reflect 
light enough for shewing an image of the candle, I was surprised 
to see that image surrounded by several waves of colours, red, 
green, and blue, disposed in pretty regular order. This was 
so uncommon in a metallic speculum, that I examined the thing 
very minutely by a variety of experiments ; these I shall not 
particularly now describe, but give a general idea of their re- 
sults. 
It must be observed, for the sake of clearness, that in the fol- 
lowing inquiries concerning the formation of rings or fringes, 
the diameter of a ring or fringe means the line passing through 
the centre of that ring, and terminated at both ends by the cir- 
cumference ; whereas the breadth means that part of the dia- 
meter intercepted between the limits of the ring, or the distance 
between its extreme colours, red and violet. 
In the first place, they were formed by the sun's light in the 
figure of rings surrounding the centre of the sphere to which 
the plate was ground, at greater distances increasing their 
breadths, the colours pretty bright, though inferior in brilliancy 
to those of concave specula. 
Secondly, the order of the colours was in all red outermost, 
and violet or blue innermost, with a greyish-blue spot in the 
common centre of the whole ; and on moving the plate from 
the perpendicular position, the rings moved and broke exactly 
like those of specula. 
In the third place, homogeneal light made them of simple 
colours ; they were broadest when red, narrowest when blue 
and violet. 
Fourthly, they decreased in breadth from the centre; and I 
mdccxcvil 3 B 
