9 
relative to physical Optics. 
in directions parallel to each other ; and these would exhibit a 
continued diffusion of fainter light, for 25 0 within the bright 
termination which forms the rainbow, but for the general law 
of interference, which, as in other similar cases, divides the light 
into concentric rings ; the magnitude of these rings depending 
on that of the drop, according to the difference of time occupied 
in the passage of the two portions, which thus proceed in parallel 
directions t,o the spectator’s eye, after having been differently 
refracted and reflected within the drop. This difference varies 
at first, nearly as the square of the angular distance from the 
primitive rainbow : and, if the first additional red be at the dis- 
tance of 2 0 from the red of the rainbow, so as to interfere a little 
with the primitive violet, the fourth additional red will be at a 
distance of nearly 2 0 more ; and the intermediate colours will 
occupy a space nearly equal to the original rainbow. In order 
to produce this effect, the drops must be about Ag- of an inch, or 
.013, in diameter : it would be sufficient if they were between 
— ^ 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 some- 
times happen, is not in itself at all improbable: we measure 
even medicines by dropping 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 determined from Dr. Langwith’s own words. 
, “ August the 21st, 1722, about half an hour past five in the 
“ evening, weather temperate, wind at north-east, the appearance 
“ was as follows. The colours of the primary rainbow were as 
“ usual, only the purple very much inclining to red, and well 
mdccciv. C 
