distempers affecting the sensations and motions, it is usual to find some great disorder in the brain, from preternatural tumours, from blood, matter, or serum, lying upon the brain, or in its ventricles, &c. This may suffice as general evidence for the present. The particular reasons of some of these phænomena, with more definite evidences, will offer themselves in the course of these observations.
The evidence for this proposition is also to be taken from the writings of physicians and anatomists; but especially from those parts of these writings which treat of the faculties of memory, attention, imagination, &c. and of mental disorders. It is sufficiently manifest from hence, that the perfection of our mental faculties depends upon the perfection of this substance; that all injuries done to it affect the trains of ideas proportionably; and that these cannot be restored to their natural course till such injuries be repaired. Poisons, spirituous liquors, opiates, fevers, blows upon the head, &c. all plainly affect the mind, by first disordering the medullary substance. And evacuations, rest, medicines, time, &c. as plainly restore the mind to its former state, by reversing the foregoing steps. But there will be more and more definite evidence offered in the course of these observations.
This is very evident in the sensations impressed on the eye. Thus, to use Sir Isaac Newton’s words, “If a burning coal be nimbly moved round in a circle, with gyrations continually repeated, the whole circle will appear like fire; the reason of which is, that the sensation of the coal, in the several places of that circle, remains impressed on the sensorium until the coal return again to the same place. And so in a quick consecution of the colours,” (viz. red, yellow, green, blue, and purple, mentioned in the experiment, whence this passage is taken,) “the impression of every colour remains on the sensorium until a revolution of all the colours be completed, and that first colour return again. The impressions therefore of all the successive colours are at once in the sensorium—and beget a sensation of white.” Opt. b. I. p. 2. Experiment 10.
Thus also, when a person has had a candle, a window, or any other lucid and well-defined object, before his eyes for a con-
