'th^ Cause of coloured coTwentr'w Rings, SOS- 



over a space of '14 of an inch, should suffer more than seven 

 interruptions of shape and colour iu the direction of the; 

 wedge of air.. .^^.^..y/ 



In order to ascerUin whether such an effect had any ex^ 

 i^tence, I viewed the secondary set of rings upon every part 

 of the glass-plate, hy moving the convex lens from one end 

 of it gradually to the other; and my attention being parti- 

 cularly directed to the 3d, 4th, and 5th rings, which were 

 extremely distinct, I saw them retain their shape and colour 

 all the time without the smallest alteration. 



The same experiment was repeated with a piece of plain 

 glass instead of the metalline mirror, in order to give rooni 

 for the fits of easy transmission, if they existed, to exert 

 themselves; but the result was still the same ; and the con- 

 stancy of the brightness and colours of the rings of the 

 secondary set, plainly proved that the rays of light were not 

 affected by the thickness of the plate of air through which 

 they passed. 



XXXI T. Alternate Fits of easy Reflection and easy Trans^ 

 " mission, if' they exist, do not exert themselves according 

 to various Thicknesses of thin Plates of Glass. 

 I selected a well polished plate of coach-glass 1 7 inches 

 long, and about 9 broad. Its thickness at one end was 33, 

 and at the other 31 two-hundredths of an inch ; so that in 

 its whole length it differed -J-^ of an inch in thickness. By 

 measuring many other parts of the plate I found that it was 

 very regularly tapering from one end to the other. This 

 plate, with a double convex lens of 55 inches laid upon it, 

 being placed upon a small metalline mirror, and properly 

 exposed to the light, gave me the usual two sets of rings. 

 In the secondary set, which was the object of my attention^ 

 I counted twelve rings, and estimated the central space be- 

 tween th^pi to be about J -^ times as hroacj as the space taken 

 up by the 1 2 rings on either side ; the whole of the space 

 taken up may therefore be reckoned equal to the breadth of 40 

 rmss of a niean size : for the 12 rings, as usual, were gra- 

 dufuly contracted in breadth as they receded from the centre j 

 and, by a measure gf the whole space thus taken up, I 



found 



