SOS ^xp^iments/orinvestigaimg 



have been transmitted will be absorbed by the metal, we 

 may admit the elusion ; it ought however to have bten made 

 a part of the hypothesis. 



XXXI. Alternate Fits of easy Reflection and easy Trans- 

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

 to various Thicknesses of thin Flates of Air, 

 In the following experiment, I placed a plain well po- 

 lished piece of glass 3*6 inches long, and 2*3 thick, upon a 

 plain metalline mirror of the same length with the glass ; 

 and in order to keep the mirror and glass at a distance from 

 each other, I laid between them, at one end, a narrow 

 strip of such paper as we commonly put between prints. 

 The thickness of -that which I used was the 6l0th part of 

 an inch; for 128 folds of them laid together would hardly 

 make up two-tenths. Upon the glass I put a 39-inch 

 double convex lens ; and having exposed this 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 

 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 amount^ 

 to the y-ir4<3ir P^i't of an inch, of which he says *' Haec est 

 crassitudo aeris in primo annulo obscure radiis ad perpendi- 

 culum incidentibus exhibito, 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 -J^ of an inch 

 from the contact ; and therefore at -^, -^, -^^, ^, -^f^ ^-|-, 

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

 and glass, equal to tW^^, TT-^oins-^ -rr^'oinr* tt^oo o> &c. 

 of which the same author says that they give ^^crassitudines 

 aeris in omnibus annulis lucidis, qua parte illi lucidissimi 

 sunt.'' Hence it follows that, according to the above hy- 

 pothesis, the rings of the secondary set which extended 

 8 over 



