i66 Mr. Knox on some phenomena of colours, 
prism, about 5^ inches long, and a full inch broad ; for al- 
though the sides had been intended to be made perfectly flat, 
they had acquired, in polishing, such a small degree of con- 
vexity, as on application of the flat glass plate, produced a 
larger set of primary rings than could be had by any of the 
former experiments. This apparatus was still farther im- 
proved, by painting two of the sides of the prism black, so as 
to exclude all extraneous light ; by which means I could easily 
perceive that those fringes (which were produced by rings 
of about three quarters of an inch diameter,) extended from 
one end of the prism to the other, that is, to an extent of at 
least seven or eight times the diameters of the rings, and it 
is uncertain how much farther they might have extended with 
a longer prism. They were also seen parallel to the edges 
of the prism, by carrying the eye in that direction, either to 
the right or left. 
Nor do these rings or fringes cross each other undisturbed, 
for the prismatic colours of the rings, where they intersect each 
other, are deranged from their natural order ; and the divi- 
sion lines of the fringes, where they intersect the other inter- 
sections, are drawn into a zig-zag form, but beyond those 
intersections they proceed in straight lines. These several 
intersections produce a most beautiful appearance of chequer, 
or rather net work, the meshes of which assume the hexagonal 
form, resembling the cups of a honeycomb; of which it is 
impossible to convey an adequate idea without seeing the ex- 
periment. 
Exp . 6. Having discovered unexpectedly these phenomena 
from one set of primaries and its reflected image, I was in- 
duced to try what effect could be produced by two sets of 
