exhibited by thin plates. 1 73 
nally in the direction of the light, and by using the shadow 
of a black card thrown across them, both primaries and trans- 
mitted fringes may be seen at once by the naked eye. Let 
the light come in the direction AB in fig. 5, pi. VIII. and let CD 
represent the edge of the shadow next the light, those fringes 
above the line CD are primaries, and those below it are trans- 
mitted sets, alternate in colours to the others. Although 
these primary fringes are uniform in their breadths, they are 
not quite so with respect to colour ; for the yellow, blue, 
indigo, and violet, are predominant in the first, second, and 
third fringes next to the place of contact, although these four 
colours are but little perceived after the third fringe, where 
ten are seen, the red, orange, and green being predominant 
in all the rest. 
Exp. 13. After having produced one set of primary fringes, 
I succeeded in having another set formed at right angles 
thereto, by applying a third slip of glass longitudinally to the 
upper one of the first two, on which the expected diagonal 
fringes immediately appeared in the angle between the two 
primary sets, as shown in fig. 6, pi. VIII. where B and C are the 
primary fringes, and D the intersectionary set divided into 
two classes, as shown by the dotted line. 
Exp. 14. It was found by trials, that the relative position of 
two sets of fringes, when formed by three slips only of glass 
plates, connected together as in Experiment 12, could not be 
changed at pleasure ; but, by using four slips, and having a 
set of fringes formed between each pair, which were uncon- 
nected, these could be placed the one over the other in any 
position, and at any required angle ; by this arrangement, it 
was discovered, that whatever was the magnitude of the angle 
