exhibited by thin plates. 167 
primaries brought into a similar situation with respect to each 
other. For this purpose, a double convex lens, of about thirty- 
six inches focus, was laid on a flat piece of looking-glass plate, 
having its under side painted black ; on the lens was placed 
another piece of plane glass plate ; by these means two sets 
of primary rings were produced, whose positions with respect 
to each other could be varied at pleasure. On using the sha- 
dow of the black card I was agreeably surprized to find, that 
instead of parallel fringes a new species of prismatic rings ap- 
peared, whose number and sizes varied with the positions and 
distances of the two sets of primaries ; their dimensions were 
from two to three times the diameters of the primaries from 
which they appeared to originate, sometimes only one set 
appeared, sometimes two, and at other times a third very 
faintly. 
On first observing these new rings, it was found, that on 
moving the eye in a horizontal direction to the right or left, 
they sometimes moved with, and sometimes contrary to the 
motion of the eye ; others were stationary, although the eye 
moved ; also, that sometimes the prismatic colours were seen 
in the usual order, and at other times inverted ; all of which 
facts seemed not a little perplexing at that time ; but their 
causes will be better understood from what follows. 
Exp. 7. By a subsequent experiment, it was discovered, 
that those rings towards the circumference of the new sets 
had their colours always in the usual order ; but that those 
nearest the centres had their colours always inverted ; that 
the number of rings of each class were equal : that they all 
passed through the several intersections of the two primary 
sets of rings with each other, from which intersections they 
