the Cause of coloured concentric Rings, 227 
exposed this combination to a proper light, I saw two com- 
plete sets of coloured rings. 
I11 this arrangement, the rays which convey the secondary 
set of rings to the eye must pass through' a thin wedge of 
air, and if these rays are endowed with permanent fits of 
easy reflection, and easy transmission, or absorption, their 
exertion, according to Sir I. Newton, should be repeated at 
every different thickness of the plate of air, which amounts 
to the g-fTsT P art an i nc h> of which he says “ Hsec est eras- 
situdo aeris in primo annulo obscuro radiis ad perpendiculum 
incidentibus exibito, qua parte is annulus obscurissimus est/’ 
The length of the thin wedge of air, reckoned from the line 
of contact, to the beginning of the interposed strip of paper, 
is 5,2 inches, from which we calculate that it will have the 
above mentioned thickness at of an inch from the contact ; 
and therefore at -A-, f-r, -A-, &c. we shall have the 
thickness of air between the mirror and glass, equal to 
-iWooo’ TTTooo’ TTFooo’ &C ' ° f which the Same aUthor Sa y S 
that they give “ crassitudines Aeris in omnibus Annulis lucidis, 
qua parte illi lucidissimi sunt/’ Hence it follows that, accor- 
ding to the above hypothesis, the rings of the secondary set 
which extended over a space of ,14 of an inch, should suffer 
more than seven interruptions of shape and colour in the di- 
rection of the wedge of air. 
In order to ascertain whether such an effect had any ex- 
istence, I viewed the secondary set of rings upon every part 
of the glass-plate, by moving the convex lens from one end of 
it gradually to the other ; and my attention being particularly 
directed to the 3d, 4th, and 3th rings, which were extremely 
