164 Afr. Knox on some phenomena of colours , 
sections of the primary set of rings with those of its reflected 
image ; and their lengths extended to the edge of the lens 
on both sides, they were formed at right angles to the direc- 
tion of the light, and to a line joining the centres of the pri- 
mary and its reflected image ; which indeed is a necessary 
consequence of their being projected through the intersections 
of two sets of concentric rings perfectly equal in dimensions. 
See fig. 1, pi. VII. compared with fig. 3, pi. VIII. 
I was convinced that those parallel fringes consisted of 
prismatic colours ; yet with the apparatus now used, they 
were too small and too close to each other to enable me to 
perceive distinctly the order in which those colours were 
arranged, with respect to each other. As their distances from 
each other, or, which is the same thing, their breadths de- 
pended on the distances of the intersections through which 
they were projected ; it was obvious that by separating, or 
widening the distances of the latter, the parallel fringes 
would also be separated, and consequently their breadths en- 
larged. 
Exp. 3. For effecting this purpose, two modes presented 
themselves, either by increasing the dimensions of the pri- 
mary set of rings, and consequently that of its reflected image; 
or by lessening the distance between them. The former mode 
was at that time out of my power, having no lens of a longer 
focus than the one in use. The latter was effected by pro- 
curing a thinner piece of plate glass than that used in the 1st 
experiment ; and although by this apparatus the field of view 
was narrowed, the breadths of the fringes were enlarged, and 
by this means it was easily perceived, that each fringe was com- 
posed of the same prismatic colours as the Newtonian rings. 
