jQO HER9CHEL Off COtOl'RED RfXG'3* 



lens ; and having exposed tins combination to a proper light, 



I saw two complete sets of coloured rings. 



In 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 



Change of easy reflection, and easy transmission, or absorption, their 



thickness in exertion, according to Sir I. Newton, should be repeated at 



capable of af- evei T different thickness of the plate of air, which amount- 



fecting the co- to the ---g}yj part of an inch, of which he says, " Hcec est 



N/Ihypothe- crassitudo aeris in primo annulo obscuro radiis ad perpendi- 



s"»s culum ineidentibus esibito, qua parte is annulus obscuris- 



simus 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 TT of an inch 



from the contact ; and therefore at T ! T , y\, ^ 5 T » /t> tt» tt» 



&c. we shall have the thickness of air between the mirror 



and glass equal to -ry-gW* T yvinnr> ty Aoo» ttiW* &c. ; 



of which the same author says, that they give " crassitudi- 



nes aeris in omnibus annulis lucidis, qua parte illi lucidis- 



sime sunt.'* Hence it follows, that, according to the above 



occurred at hypothesis, the rings of the secondary set, which extended 



ies> over a space of *14 of an inch, should suffer more than seven 



interruptions of shape and colour in the direction of the 



wedge of air. 



In order to ascertain, whether such an effect had any e' 



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 parti- 



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



ducing the ef- extremely distinct, 1 saw them retain their shape and colour 

 feet. 



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 room 

 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 se- 

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

 affected by the thickness of the plate of air through which 

 thev passed. 



XXXIT. 



