siderable time, he may perceive a very clear and precise image thereof to be left in the sensorium, fancy, or mind (for these I consider as equivalent expressions in our entrance upon these disquisitions,) for some time after he has closed his eyes. At least this will happen frequently to persons who are attentive to these things in a gentle way; for, as this appearance escapes the notice of those who are entirely inattentive, so too earnest a desire and attention prevents it, by introducing another state of mind or fancy.
To these may be referred the appearance mentioned by Sir Isaac Newton, Opt. Qu. 16. viz. “When a man in the dark presses either corner of his eye with his finger, and turns his eye away from his finger, he will see a circle of colours like those in the feather of a peacock’s tail. And this appearance continues about a second of time after the eye and finger have remained quiet.” The sensation continues therefore in the mind about a second of time after its cause ceases to act.
The same continuance of the sensations is also evident in the ear. For the sounds which we hear are reflected by the neighbouring bodies, and therefore consist of a variety of sounds, succeeding each other at different distances of time, according to the distances of the several reflecting bodies; which yet causes no confusion or apparent complexity of sound, unless the distance of the reflecting bodies be very considerable, as in spacious buildings. Much less are we able to distinguish the successive pulses of the air, even in the gravest sounds.
As to the senses of taste and smell, there seems to be no clear direct evidence for the continuance of their sensations after the proper objects are removed. But analogy would incline one to believe, that they must resemble the senses of sight and hearing in this particular, though the continuance cannot be perceived distinctly, on account of the shortness of it, or other circumstances. For the sensations must be supposed to bear such an analogy to each other, and so to depend in common upon the brain, that all evidences for the continuance of sensations in any one sense, will extend themselves to the rest. Thus all the senses may be considered as so many kinds of feeling; the taste is nearly allied to the feeling, the smell to the taste, and the sight and hearing to each other. All which analogies will offer themselves to view when we come to examine each of these senses in particular.
In the sense of feeling, the continuance of heat, after the heating body is removed, and that of the smart of a wound, after the instant of infliction, seem to be of the same kind with the appearances taken notice of in the eye and ear.
But the greatest part of the sensations of this sense resemble those of taste and smell, and vanish to appearance as soon as the objects are removed.
