Routledge History of Philosophy Volume IV

DESCARTES: METHODOLOGY 169
from the part of the sky marked AFZ, and my eye be at point E, then when
I put this globe at the place BCD, the part of it at D seems to me wholly red
and incomparably more brilliant than the rest. And whether I move
towards it or step back from it, or move it to the right or to the left, or even
turn it in a circle around my head, then provided the line DE always marks
an angle of around 42° with the line EM, which one must imagine to
extend from the centre of the eye to the centre of the sun, D always appears
equally red. But as soon as I made this angle DEM the slightest bit smaller
it did not disappear completely in the one stroke but first divided as into two
less brilliant parts in which could be seen yellow, blue, and other colours.
Then, looking towards the place marked K on the globe, I perceived that,
making the angle KEM around 52°, K also seemed to be coloured red, but
not so brilliant… 28
Descartes then describes how he covered the globe at all points except B and D.
The ray still emerged, showing that the primary and secondary bows are caused
by two refractions and one or two internal reflections of the incident ray. He next
describes how the same effect can be produced with a prism, and this indicates
that neither a curved surface nor reflection are necessary for colour dispersion.