To say that light is invisible, however, does not mean that it is wholly imperceptible. It is difficult to bring the perception of light into consciousness, for naturally our attention, when we look out into light-filled space, is claimed by the objects of the illuminated world, in all their manifold colours and forms. Nevertheless the effect of pure light on our consciousness can be observed during a railway journey, for instance, when we leave a tunnel that has been long enough to bring about a complete adaptation of the eyes to the prevailing darkness. Then, in the first moments of the lightening of the field of vision, and before any separate objects catch the attention, we can notice how the light itself exercises a distinctly expanding influence on our consciousness. We feel how the light calls on the consciousness to participate, as it were, in the world outside the body.