exhibited by thin plates. 163 
that those parallel lines were drawn through the intersections 
made by the several rings of the primary set and its reflected 
image, and that they consisted of light, although their di- 
mensions were too small for their colours to be distinctly 
perceived. 
The apparatus used in this experiment was a piece of good 
looking-glass plate laid on the plane side of a plano-concave 
lens belonging to a compound microscope, and was used for 
holding mites or other animalcules on the concave side. The 
plane side of this glass had acquired in the polishing ( acci- 
dentally perhaps ) a very small degree of convexity, probably 
equal to that of a lens of several feet focal length. This lens, 
when the plane glass was laid thereon, produced a larger set 
of primary rings than could be otherwise procured. Its con- 
cave side had been ground on a sphere of about two inches 
radius ; therefore the set of transmitted rings, reflected from 
its lower internal surface, was too small to cause any con- 
fusion either in the primary set or its reflected image ; and it 
was in a great measure owing to this accidental circumstance, 
that these parallel lines happened to be discovered at all ; nor 
could they have been discovered, even with this apparatus, 
without the use of the shadow. 
Exp . 2. Having ascertained the reality of these parallel 
fringes, I painted the concave side of the plano-concave lens 
black, in order to prevent all reflection from its concave sur- 
face ; by which means the fringes were seen in much greater 
perfection. They were found to consist of all the prismatic 
colours; were equidistant and parallel; equal in number to 
the rings of both sets taken together, exclusive of the central 
one ; and each fringe was drawn through the several inter- 
Y 2 
