"'A kind of halo begins to gather round it: my eyes dazzle.'
"'Then raise them to the heavens; fix them on yonder blue sky. What do you see?'
"'I see it still; but it is as white as mist, and of a gigantic size.'
"'Has it a head?' asked the Moonshee in an anxious whisper.
"'Yes, it is complete in all its parts; but now it melts—floats—disappears.'
"'Thank God!' said the old man: 'your journey shall be prosperous, such is the will of Heaven.'"[50]
When a steady gaze is maintained upon an object until the retina is exhausted, which is shown by the imperfect vision, or "dazzling," and the eyes are then suddenly directed away from it to an uniformly coloured surface, an image of the object, from the persistence of the impression, as already stated, will still remain for a short period upon the retina; but another phenomenon is also observed, for the exhausted condition of the retina renders it incapable of responding, during its continuation, to the impression of the original colour of the object, and the spectrum appears of a different colour. To this spectral colour the term complementary or accidental is applied; and if the colour of the object be red, the complementary colour will be green; if yellow, deep purple; if black, white, &c., and vice versâ. Thus then the spectral apparition witnessed in the above relation receives a ready and intelligible explanation.
The sense of hearing is also subject to illusions: for example, when a timid person mistakes the rustling of leaves in a forest for the voices of robbers; or the soughing of the wind among the trees, in some place of evil repute, for the moaning of a wandering and unhappy spirit.
The varied and undefined noises often produced by the wind when sweeping over an irregular surface, among rocks and trees, on the surface of water, in forests, or secluded and deep glens; and the mysterious sounds occasioned by the rushing of the water in the hollows and caverns of a rock-bound coast, have been fertile sources of illusion among the superstitious.