37 
the Theory of Light and Colours. 
difficult to invent any other that would account for it. There 
is a striking analogy between this separation of colours, and the 
production of a musical note by successive echoes from equi- 
distant iron palisades ; which I have found to correspond pretty 
accurately with the known velocity of sound, and the distances 
of the surfaces. 
It is not improbable that the colours of the integuments of 
some insects, and of some other natural bodies, exhibiting in 
different lights the most beautiful versatility, may be found to 
be of this description, and not to be derived from thin plates. 
In some cases, a single scratch or furrow may produce similar 
effects, by the reflection of its opposite edges. 
Corollary if.. Of the Colours of thin Plates. 
'When a beam of light falls on two parallel refracting sur- 
faces, the partial reflections coincide perfectly in direction ; and, 
in this case, the interval of retardation, taken between the sur- 
faces, is to their distance as twice the cosine of the angle of 
refraction to the radius. For, in Fig. 3, drawing AB and CD 
perpendicular to the rays, the times of passing through BC and 
AD will be equal, and DE will be half the interval of retarda- 
tion ; but DE is to CE as the sine of DCE to the radius. Hence, 
that DE may be- constant, or that the same colour may be re- 
flected;, the thickness CE must vary as the secant of the angle 
of refraction CED : which agrees exactly with Newton’s expe- 
riments ; for the correction is perfectly inconsiderable. 
Let the medium between, the surfaces be rarer than the sur- 
rounding mediums ; then the impulse reflected at the second 
surface, meeting a subsequent undulation at the first, will render 
the particles of the rarer medium capable of wholly stopping 
