exhibited by thin plates. 169 
the reticulated crossings before mentioned, whether the sets of 
primaries by which they are caused, are equal or unequal in 
dimensions. Fringes and intersectionary rings, therefore, 
differ in no other respect than as right lines and circles ; it is, 
therefore, a fair conclusion, that the phenomena of rectilinear 
fringes, formed between two sets of rings of equal magni- 
tudes, are bent into rings when the two corresponding sets 
are unequal in magnitude. 
There is also an infinite variety in the dimensions of those 
intersectionary rings, according as the diameters of the pri- 
maries differ more or less, being least where that difference is 
greatest, and increasing in size as the two sets of primaries 
approach to equality, until at last they end in straight lines. 
The dimensions of these intersectionaries will also ( cceteris 
paribus ) diminish as the two sets of primaries approach each 
other, and enlarge as these are separated ; and, like Newto- 
nian rings, enlarge or diminish with the less or greater ele- 
vation of the eye. It will be easy to conceive from inspection 
of fig. 4, ( pi. VIII. ) that where an intersectionary set of rings 
is formed between two sets of primaries of unequal magnitudes, 
it must necessarily appear on the side towards the smaller 
of the two primaries ; which agrees with actual experiment. 
The above hypothesis accounts for one set of intersection- 
aries ; but experiment shows that if one set appears, it is al- 
most always accompanied by a second, sometimes equal, but 
oftener unequal in dimensions, as in fig. 3, (pi. VIII.) at other 
times part of a third set appears as segments of circles only. It 
would, therefore, seem that they are formed not only between 
primary sets, but also between primaries combined with either 
MDCCCXV. Z 
