to the pupil, and of about the same size with it, in its ordinary dimensions. The last, or the edges, may answer to those parts of the retina, which are only sometimes exposed to the action of light, viz. in dilatations of the pupil. It is observable, that the central parts are often dark, while the edges are luminous; and vice versâ. It happens also frequently, that in the whole appearance a blue, a dilute yellow, and a red, succeed each other in the order of the colours. Perhaps, by farther observations, a person might be able, in some measure, to predict the variations of this phænomenon. It generally moves, which may be perhaps from the motion of the peculiar vibrations along the surface of the retina.
Upon shutting one’s eyes after they have been fixed upon a luminous object, as a candle, a fire, a window, it is common to have a faint image of the object remain in the eye for a few moments. This follows from the gradual declension of the vibrations excited.
Sometimes, instead of a luminous image, a dark one, corresponding in shape and size to the luminous object, presents itself. In this case we must suppose, that the vivid vibrations excited by the luminous object pass immediately into very feeble ones, absolutely or relatively, upon the cessation of the impression.
Boerhaave says, that he had a luminous circle in his eye for a long time after having viewed too intensely the moon’s light collected to a focus. And looking at the sun makes other objects appear red and luminous. It does also occasion dark spots to appear upon common objects afterwards. This last effect may perhaps succeed the first. While extraordinary vibrations, or a disposition to them, continue in the retina, and optic nerve (which may be for a longer time, if a slight inflammation, with the consequent irritability, be produced in the small vessels of the nervous capillaments), the common objects may appear luminous. When these go off, a contrary state may take place, and cause the dark spots to appear. Dark spots of continuance argue, that an injury is done to the retina, and optic nerve. The permanent dark spots, which are sometimes previous to a gutta serena, seem to be of this kind.
It is agreeable to some of the foregoing instances, that being kept much in the dark should enable the persons to see with a very obscure light. In some other cases of a nyctalopia there may perhaps be the first and lowest degree of inflammation in the infinitesimal vessels of the retina, so as to increase the sensibility of the organ without making the exercise of its functions painful.
Giddiness, or an apparent irregular motion in the objects of sight, almost always goes before any general confusion and privation of sense and motion; which is very agreeable to the doctrine of vibrations. For the general disorder in the vibrations in the medullary substance may be expected to be perceived in the optic nerve, and corresponding part of the brain, first and chiefly, on
