644 



iXPEHlMENTS AVD CAJXUXATIONS 



<tises, divides the light into concentric 

 rings; the magnitude of these rings de- 

 pending on that of the drop, according to 

 the difference of time occupied in the pas- 

 sage of the two portions, which thus proceed 

 in parallel directions to the spectatoi-'s eye, 

 after having been differently refracted and 

 reflected within the drop. This difl'erence 

 varies at first, nearly as the square of the an- 

 gular distance from the primitive rainbow: and, 

 if the first additional red be at the distance 

 of 2° from the red of the rainbow, so as to 

 interfere a little with the primitive violet, the 

 fourth ad.litional red will be at the distance 

 of nearly 2° more ; and the intermediate co- 

 lours will occupy a space nearly equal to the 

 original rainbow. In order to produce this 

 effect, the drops must be about ^ of an inch, 

 or .013, in diameter: it would be sufficient 

 if they were between yV and^. The reason, 

 that such supernumerary colours are not often 

 seen, must be , that it does not often happen 

 that drops so nearly equal are found together: 

 but, that this may sometimes happen, is not 

 in itself at all improbable: we measure even 

 medicines by dro[>ping them from a phial, 

 and it may easily be conceived that the drops, 

 formed by natural operations, may sometimes 

 be as uniform, as any that can be produced 

 by art. How accurately this theory coincides 

 with the observation, may best be deter- 

 mined from Dr. Langwith's own words. 



"August the 21st, 1722, about half an 

 hour past five in the evening, weather tem- 

 perate, wind at north east, the appearance 

 was as follows. The colours of the primary 

 rainbow wer-e as usual, 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 dusky green : under this 

 were alternately two arches of reddish purple, 

 and two of green : under all, a faint appear- 

 ance of another arch of purple, which va- 

 nished and returned several times so quick, 

 that \jc could not readily fix our eyes upon 

 it. Thus the order of the colours was, i. Red, 

 orange colour, yellow, green, light blue, deep 

 blue, purple, n. Light green, dark green, 

 purple, in. Green, purple. <v. Green, faint 

 vanishing purple. You see" we had here foiu- 

 orders of coloui-s, and perhaps the beginning 

 of a fifth : for I make no question but that 

 what I call the purple, is a mixture of the 

 purple of each of the upper series with the 

 red of the next below it, and the green a 

 mixture of the intermediate colours. I send 

 you not this account barely upon the credit 

 of my own eyes ; for there was a clergyman 

 and fourother gentlemen in company, whom 

 1 desired to view the colours attentively, who 

 all agreed, that they appeared in the manner 

 that I h ave now described . There are two things 

 which well deserve to be taken notice of, as 

 they may perhaps direct us, in some mea- 

 sure, to the solution of this curious pheno- 

 menon. The first is, that the breadth of the 

 first series so far exceeded that of any of the 

 rest, that, as near as I could judge, it was 

 equal to thein all taken together. The se- 

 cond is, that I have never observed these in- 

 ner orders of colours in the lower parts of the 

 rainbow, though they have often been incom- 

 parably more vivid than the upper parts, un- 

 der which the colours have appeared. I have 

 taken notice of this so very often, that 1 can 

 hardly look upon it to be accidental; and, 

 if it should prove true in general, it will bring 

 the disquisition into a narrow compass ; for 

 it will show that this effect depends upon 



