13« 



Application of 

 the doftrine of 

 the interference 

 of light to the 

 fupcrnumerary 

 rainbows of 

 Langwith and 

 D4v4. 



rirysicAt optics. 



more ; and the intermediate colours will occupy a fpace nearly 

 equal to the original rainbow. In order to produce this effed, 

 the drops mull be about -^ <^ an inch, or .013, in diameter : 

 it would be fufficient if they were between -jq and -3%. The 

 reafon that fuch fupernunierary colours are not often feen, 

 mufl be, that it does not often happen that drops fo nearly 

 equal are found together: but, that this may fometimes 

 happen, is not in itfelf at all improbable : we meafure even 

 medicines by dropping them from a phial, and it may eafily 

 be conceived that the drops formed by natural operations may 

 fometimes be as uniform as any that can be produced by art.' 

 How accurately this theory coincides with the obfervation, 

 may beft be determined from Dr. Lan<>with's own words. 



** Auguft the 2 1 ft, 1722, about half an hour paft five in the 

 evening, weather temperate, wind at north-eafl, the appear- 

 rance was as follows : The colours of the primary rainbow 

 were as ufual, only the purple very much inclining to red, 

 and well defined : under this was an arch of green, the upper 

 part of which inclined to a bright yellow, the lower to a more 

 dullvy green : under this were alternately two arches of reddifli 

 purple, and two of green : under all, a faint appearance of 

 another arch of purple, which vaniflied and returned feveral 

 limes fo quick, that we could not readily fix our eyes upon it. 

 Thus the order of the colours was, i. Red, orange-colour, yel- 

 low, green, light blue, deep blue, purple, 11. Light green, 

 dark green, purple, ui. Green, purple, jv. Green, faint 

 vanifiiing purple. You fee we had here four orders of colours, - 

 and perhaps the beginning of a fifth ; for I make no queftion 

 but that what I call the purple, is a mixture of the purple of 

 each of the upper feries with the red (f the next below it, and 

 the green a mixture of the intermediate colours. I fend you not 

 this account barely upon the credit of my own eyes; for there 

 was a clergyman and four other gentlemen in company, whom 

 I defired to view the colours attentively, who all agreed, 

 that they appeared in the manner that I have now defcribed. 

 There are two things which well deferve to be taken notice of, 

 as they may perhaps dired us, in fome meafure, to the folution 

 of this curious phenomenon. Tlie firft is, that the breadth of 

 the firft feries fo far exceeded that of any of the rell, that, as 

 near as I could judge, it was equal to them all taken together. 

 The fecond is, that I have iiever obferved thefe inner orders 



of 



