lys Mr. Knox on some phenomena of colours, 
would prevent the success of the experiment), and by apply- 
ing two of their ends together, and by using some friction and 
a considerable degree of pressure, a beautiful set of rectilinear 
prismatic coloured fringes appeared across the glass slips, 
having all the prismatic colours in the same order as Newtonian 
rings, and equally vivid, (see fig. 5, pi. VIII.) and although 
the breadths of those fringes could be increased or diminished 
by the greater or less degree of pressure (which had the 
effect of diminishing or increasing the angle formed by the 
planes), yet their breadths continued to be uniform, or as 
nearly so as might be expected, considering the unavoidable 
imperfections of the best plate glass. 
It was proved by Sir Isaac Newton, that primary prismatic 
rings were repeated at equal increments of interval between 
two spherical surfaces, because the semi-diameters (or, which 
is the same thing, the diameters) of those rings were found 
on actual measurement to be to each other as the square roots 
of the series, o, 2, 4, 6 , 8, &c. beginning at the centre of the 
rings or point of contact, as must be well known to mathema- 
ticians ; it might, therefore, have been known by reasoning a 
priori, that if similar prismatic phenomena could be produced 
between two flat planes, the corresponding repetitions must 
take place at regular and equal intervals from the point of 
contact, or vertex of the angle, corresponding to the natural 
numbers of which the measurements of the rings express 
the square roots ; that is, as the series o, 2, 4, 6 , 8, &c. The 
principle is, therefore, fully confirmed by this experiment. 
Exp. 12. Those primary fringes have also transmitted sets 
between them, alternate in colours to their primaries, exactly 
similar to Newtonian rings ; for, by placing them Iongitudi- 
