On the Unequal Sensibility of the Foramen Centrale to Light of
different Colours.

James Clerk Maxwell


[From the _Report of the British Association_, 1856.]


When observing the spectrum formed by looking at a long vertical slit
through a simple prism, I noticed an elongated dark spot running up
and down in the blue, and following the motion of the eye as it moved
_up and down_ the spectrum, but refusing to pass out of the blue into
the other colours. It was plain that the spot belonged both to the
eye and to the blue part of the spectrum. The result to which I have
come is, that the appearance is due to the yellow spot on the retina,
commonly called the _Foramen Centrale_ of Soemmering. The most
convenient method of observing the spot is by presenting to the eye in
not too rapid succession, blue and yellow glasses, or, still better,
allowing blue and yellow papers to revolve slowly before the eye. In
this way the spot is seen in the blue. It fades rapidly, but is
renewed every time the yellow comes in to relieve the effect of the
blue. By using a Nicol's prism along with this apparatus, the brushes
of Haidinger are well seen in connexion with the spot, and the fact of
the brushes being the spot analysed by polarized light becomes
evident. If we look steadily at an object behind a series of bright
bars which move in front of it, we shall see a curious bending of the
bars as they come up to the place of the yellow spot. The part which
comes over the spot seems to start in advance of the rest of the bar,
and this would seem to indicate a greater rapidity of sensation at the