171 
exhibited by thin plates. 
matic rings in figures 1, 2, 3, (plates VII. VIII.) are such as 
would appear to the eye, could it see them at right angles to the 
plane in which they are formed ; but as the eye cannot well 
see them in this position, nor until it is lowered to an obliquity 
of about thirty degrees, the proper allowance must be made ; 
for such rings will always appear of elliptical forms, less or 
more elongated according to the greater or less elevation of 
the eye. See fig. 4. (pi. VIII.) 
Exp. 10. Finding from the foregoing experiments, that two 
sets of rings of equal magnitudes produced straight fringes, 
and that those of unequal sizes produced rings ; it was obvious 
that both fringes and rings might be considered as diagonals 
to the angles in which they were formed. It was, therefore, 
concluded from analogy, that if primary fringes could be pro- 
duced between two flat pieces of looking-glass plate, and 
that if two of those sets were made to cross each other, a set 
of straight or rectilinear fringes should be formed diagonally 
between them ; and, on making the experiment, I had the 
pleasure to find the result precisely answered expectation. So 
far theory and experiment agreed ( see fig. 6, pi. VIII. ) ; but 
in the course of making this and some of the succeeding 
experiments, several unexpected phenomena were discovered. 
Exp. 11. The slips of plate glass used in this and several 
of the following experiments were from four to six inches 
long, and about one and a half inch broad, their edges having 
been ground straight. 
On wiping two of those glass slips perfectly clean with 
soft shammy or a soft dry linen rag, (wdiich is absolutely 
necessary, as the smallest particle of dust, scarcely percepti- 
ble even with a ^microscope, interposed between the slips, 
Z 2 
