When medical men are called upon to attend a commission of lunacy, they are always asked, whether the patient has had a _lucid interval_? A term of such latitude as _interval_ requires to be explained in the most perspicuous and accurate manner. In common language it is made to signify, both a moment and a number of years, consequently it does not comprize any stated time. The term _lucid interval_ is therefore relative. I should define a _lucid interval_ to be _a complete recovery of the patient's intellects, ascertained by repeated examinations of his conversation, and by constant observation of his conduct, for a time sufficient to enable the superintendant to form a correct judgment_. Unthinking people are frequently led to conclude that, if during a conversation of a few minutes, a person under confinement shall betray nothing absurd or incorrect, he is well, and often remonstrate on the injustice of secluding him from the world. Even in common society, there are many persons whom we never suspect from a few trifling topics of discourse to be shallow minded; but, if we start a subject, and wish to discuss it through all it's ramifications and dependances we find them incapable of pursuing a connected chain of reasoning. In the same manner, insane people will often, for a short time, conduct themselves, both in conversation and behaviour, with such propriety, that they appear to have the just exercise and direction of their faculties; but let the examiner protract the discourse, until the favourite subject shall have got afloat in the madman's brain, and he will be convinced of the hastiness of his decision. To those unaccustomed to insane people, a few coherent sentences, or rational answers would indicate a lucid interval, because they discover no madness; but he who is in possession of the peculiar turn of the patient's thoughts, might lead him to disclose them, or by a continuance of the conversation they would spontaneously break forth. A beautiful illustration of this is contained in the Rasselas of Dr. Johnson: where the astronomer is admired as a person of sound intellect and great acquirements by Imlac, who is himself a philosopher, and a man of the world. His intercourse with the astronomer is frequent; and he always finds in his society information and delight. At length he receives Imlac into the most unbounded confidence, and imparts to him the momentous secret. "Hear Imlac what thou wilt not without difficulty credit. I have possessed for five years the regulation of weather, and the distribution of the seasons. The sun has listened to my dictates, and passed from tropic to tropic by my direction. The clouds at my call have poured their waters, and the Nile has overflowed at my command. I have restrained the rage of the Dog-star, and mitigated the fervours of the Crab. The winds alone of all the elemental powers have hitherto refused my authority, and multitudes have perished by equinoctial tempests, which I found myself