HERSCHEL ON COLOURRD RINGS. ]Q3 



XXXII. 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 17 inches No fits exert 

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

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

 its whole length it differed T ^ of an inch in thickness. By "f g ^° f P 

 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 them to be about V± times as broad as the space taken 

 up by the 12 rings on either side ; the* whole of the space 

 taken up may therefore be reckoned equal to the breadth of 

 40 rings of a mean size: for the 12 rings, as usual, were 

 gradually contracted in breadth as they receded from the 

 centre, and, by a measure of the whole space thus taken up, 

 1 found, that the breadth of a ring of a mean size was about 

 the 308th part of an inch. 



Now, according to Sir I. Newton's calculation of the ac- 

 tion of the fits of easy reflection and easy transmission in 

 thick glass plates, an alternation from a reflecting to a trans- 

 mitting fit requires a difference of tttttt P art pf an inch in 

 thickness* ; and by calculation this difference took place in 

 the glass plate that was used at every 80th part of an inch 

 of its whole length; the 12 rings, as well as the central co- 

 lour of the secondary set, should consequently have been 

 broken by the exertion of the fits at every 80th part of an 

 inch ; and from the space over which these rings extended, 

 which was about *13 inch, we find that there must have been 

 more than ten such interruptions or breaks in a set of which 

 the 308th part was plainly to be distinguished. Put when 

 I drew the glass pjate gently over the small mirror, keeping . 



# Newton's Optics, p 277. 



Vol. XIX— March, 1808. O the 



