392 Remarks on a Paper by Dr Scoresby. 
shadow of the observer. The colours are always the reverse 
of those of the rainbow. Such phenomena have been for- 
merly described by Dr Scoresby himself.* 
The question then comes to be, (1.) how were the rainbow 
colours perceived at varying angles in the case of the dew- 
drop, whereas in the rainbow the angle is nearly constant? 
(2.) how come the two sets of phenomena to be nearly mixed 
up together, so that it was impossible to tell where the one be- 
gins and the other ends? In a word, this arose entirely from 
the smaliness and from the varying size of the rain drops. The 
phenomena described by Dr Scoresby are precisely what any 
one acquainted with the modern theory of the rainbow found- 
ed by Dr Young would have predicted, and which indeed form 
but a particular case of M. Babinet’s very pretty experiment 
of shewing how the radius of any coloured arch of a rainbow 
produced in a fine cylindric streamlet of water varies as the 
diameter of the stream alters. The diameter even of the pri- 
mary rainbow varies with the size of the drops (Report on Me- 
teorology, p. 129), so that each drop produces a spectrum of 
its own at an angular distance from the sun depending upon 
the drop’s diameter. But besides this, each drop produces its 
supernumerary bows, which are of greater distinctness as the 
drops are smaller, and of which M. Babinet has had the pa- 
tience to count sixteen interior and nine exterior repetitions 
of colour. 
The colours described by Dr Scoresby, even for the bows 
of 40°—50°, are not pure, since from the minuteness of the 
drops the interference of the light arriving at the eye from dif- 
ferent parts of the same drop determines their angular mea- 
sure, so whenever interference enters there must be mixed 
colours depending upon superpositions, as in Newton’s rings. 
There were particular positions at which Dr Scoresby saw 
most light of any colour, which correspond to the dimension 
of the rainbow for the mean sized drop. 
The full range of colour was only seen in drops fifteen or 
twenty yards off (p. 52), whilst those much nearer the eye 
shewed only three colours. The explanation is, that the de- 
* Sce the Report above cited, p. 138. 
