(I) The retina, the screen at the back of the eye on which
the lens of the eye casts its image, contains two
different kinds of sensitive elements. (II) One set, the
rods, register shape; the other, the cones, register
colour. (III) Animals that sleep at night and are active
mostly during the daylight hours have a mixture of both
of these in their retinas. (IV) Some species are able to
see over an even wider colour spectrum than we can.
(V) But colour is largely invisible at night so nocturnal
creatures have eyes with retinas packed almost entirely
with rods.