This play of the organs, (which how ever is rather to be referred to the ex ternal than to the mental organs), gives rise, in the case of vision, to a number of very singular and interesting pheno mena, by some philosophers called ocular spectra. A considerable variety of them are stated by Dr. R. Darwin, of Shrews bury, at the end of the second part of Darwin's Zoonomia. We shall select a few of the most striking.
Place about half an inch square of white paper on a black hat, and looking steadi ly on the centre of it for a minute, re move your eyes to a sheet of white pa per ; after a second or two a dark square will be seen on the white paper, which will be seen for some time. A similar dark square will be seen in the closed eye, if light be admitted through the eye lids. So, after looking at any lumi nous body of a small size, as at the Sun, for a short time, so as not much to fatigue the eyes, this part of the retina becomes less sensible to smaller quantities of light: hence, when the eyes are turned upon other less luminous parts of the sky, a dark spot is seen, resembling the shape of the luminous body. 'l'o the same cause Dr. R. Darwin ascribes those dark coloured floating spots, which are easily perceptible when the eyes are a little weakened by fatigue, and during ill nesses which are attended with great debility. He says, that as these spec tra are most easily discernible when our eyes are weakened by fatigue, it has frequently happened that people of de licate constitutions have been much alarmed at them, fearing a beginning decay of their sight, and thence have fallen into the hands of ignorant ocu lists. They are not, however, he ob serves, the preludes to any disease, and it is only from our habitual inattention to them that we do not see them on all objects every hour of our lives. As the nerves of very weak people, he continues, lose their sensibility by a small duration of exertion, it frequently happens that sick people, in the ex treme debility of fevers, are perpetual ly employed in picking something off from the bed clothes, owing to their mistaking the cause of these dark spots. An Italian artist, a man of strong abilities, relates, that having passed the whole night on a distant mountain, with some companions and a eoujuror, and performed many ceremonies to raise the devil, on their return in the morning to Rome, looking up when the sun began to rise, they saw numerous devils run on the tops of the houses as they passed along. So much were the spectra of
their weakened eyes magnified by fear, and made subservient to the purposes of fraud or superstition.
Again, make with ink, on white paper, a very black spot about half an inch in diameter, with a tail about an itch in length, so as to represent a tadpole. Look steadily at this spot for about a mi nute, and on moving the eye a little, the figure of the tadpole will be seen on the white part of the paper, which figure will appear whiter or more luminous than the other part of the paper. This Dr. R Darwin brings as one proof; that when the retina has been subjected to less excitement, it is more easily brought into action by being subjected to a greater. A surface appears black in consequence of its absorbing all the rays of light ; that part of the retina, therefore, which is unemployed while looking at the spot, is afterwards More sensible of the light from the white pa per, than those parts which had previ ously been exposed to it. On closing the eyes after viewing the black spot on the white paper, a red spot is seen of the form of the black spot ; for that part of the retina on which the figure of the black spot was formed, being more sensible to the light than the other parts, is capable of being brought into action by the red rays which pene trate the eye-lids. Upon the same prin ciple Dr. R. Darwin accounts for the fol lowing fact. A. writer in the Berlin Me moirs observes, that when he held a book, so that the sun shone upon his half closed eye-lids, the black letters which he had long inspected, became red. There is a similar story told by Voltaire of a Duke of Tuscany, who was playing at dice with a general of a foreign 'army, and believing that he saw red spots on the dice, portended dread events, and retired in confusion. 'the observer, after looking for a minute on the black spots of a die, in a bright day, and carelessly closing his eyes, would see red spots corresponding to the black spots on the (lie, and if they were intense, from the fatigue or weak ness of the optic organ, those appearances would continue, and on looking at the die, would be supposed to be upon it, just as before stated ; persons in a very weak state often see black spots which they refer to the bed clothes.