Dr. R. Darwin seems to believe, that it is from habit only, and want of attention, that we do not see the remains of former impressions, or the _muscæ volitantes_, on all objects.[2] Probably, this is an instance, in which the error of external sensation is corrected by experience, like the deceptions of perspective, which are undoubtedly strong in our childhood, and are only detected by repeated observation. “After having looked,” says Dr. Darwin, “long at the meridian sun, in making some of the preceding experiments, till the disk faded into a pale blue, I frequently observed a bright blue spectrum of the sun in other objects all the next and the succeeding day, which constantly occurred when I attended to it, and frequently when I did not attend to it. When I closed and covered my eyes, this appeared of a dull yellow; and at other times mixed with the colours of other objects on which it was thrown.”[3] It is scarcely necessary to mention the well-known experiment of giving a rotatory motion to a piece of burning wood, the effect of which is to exhibit a complete fiery circle to the eye. To this principle of a renewal of impressions formerly made by different objects, belongs the idle amusement of tracing landscapes, and pictures of various composition, in the discoloured spots of an old wall. This may be truly called a waking dream, as it is composed of the shreds and patches of past sensations; yet there are, perhaps, few persons who have not occasionally derived entertainment from it. It is probably on the same principle, that we are to account for the appearances of armies marching, in desert and inaccessible places, which are sometimes beheld by the inhabitants of the vallies, in mountainous regions. The accidents of light and shade, and the interposition of partial fogs, or clouds, produce the same effect on the eye, as the discoloured patches of the wall; and the rolling of the mist adds motion to the spectral images.