LUCID INTERVAL, med. jur. That space of time between two fits of insanity, 
during which a person non compos mentis is completely restored to the 
perfect enjoyment of reason upon every subject upon which the mind was 
previously cognizant. Shelf. on Lun. 70; Male's Elem. of Forensic Medicine, 
227; and see Dr. Haslam on Madness, 46; Reid's Essays on Hypochondriasis, 
317 Willis on Mental Derangement, 151. 
     2. To ascertain whether a partial restoration to sanity is a lucid 
interval, we must consider the nature of the interval and its duration. 1st. 
Of its nature.: "It must not," says D'Aguesseau, "be a superficial 
tranquillity, a shadow of repose, but on the contrary, a profound 
tranquillity, a real repose; it must not be a mere ray of reason, which only 
makes its absence more apparent when it is gone, not a flash of lightning, 
which pierces through the darkness only to render it more gloomy and dismal, 
not a glimmering which unites night to the day; but a perfect light, a 
lively and continued lustre, a full and entire day, interposed between two 
separate nights of the fury which precedes and follows it; and to use 
another image, it is not a deceitful and faithless stillness, which follows 
or forebodes a storm, but a sure and steady tranquillity for a time, a real 
calm, a perfect serenity; without looking for so many metaphors to represent 
an idea, it must not be a mere diminution, a remission of the complaint, but 
a kind of temporary cure, an intermission so clearly marked, as in every 
respect to resemble the restoration of health." 2d, Of its duration. "As it 
is impossible," he continues, "to judge in a moment of the qualities of an 
interval, it is requisite that there should be a sufficient length of time 
for giving a perfect assurance of the temporary reestablishment of reason, 
which it is not possible to define in general, and which depends upon the 
different kinds of fury, but it is certain there must be a time, and a 
considerable time." 2 Evan's Poth. on Oblig. 668, 669. 
     3. It is the duty of the party who contends for a lucid interval to 
prove it; for a person once insane is presumed so, until it is shown that he 
has a lucid interval or has recovered. Swinb. 77; Co. Litt. by Butler, n. 
185; 3 Bro. C. C. 443; 1 Rep. Con. Ct. 225; 1 Pet. R. 163; 1 Litt. R. 102. 
Except perhaps the alleged insanity was very long ago, or for a very short 
continuance. And the wisdom of a testament, when it is proved that the party 
framed it without assistance, is a strong presumption of the sanity of a 
testator. 1 Phill. R. 90;1 Hen. & Munf. 476. 
     4. Medical men have doubted of the existence of a lucid interval, in 
which the mind was completely restored to its sane state. It is only an 
abatement of the symptoms, they say, and not a removal of the cause of the 
disease; a degree of irritability of the brain remains behind which renders 
the patient unable to withstand any unusual emotion, any sudden provocation, 
or any unexpected pressing emergency. Dr. Combe, Observations on Mental 
Derangement, 241; Halsam, Med. Jur. of Insanity, 224; Fodere, De Medecine 
Legale, tom, 1, p. 205, 140; Georget, Des Maladies Mentales, 46; 2 Phillim. 
R. 90; 2 Hagg. Eccl. R. 433; 1 Phillim. Eccl. R. 84. 
     See further, Godolph. 25; 3 Bro. C. C. 443; 11 Ves. 11; Com. Dig. 
Testimoigne, A 1; 1 Phil. Ev. 8; 2 Hale, 278; 10 Harg. State Tr. 478; 
Erskine's Speeches, vol. 5, p. l; 1 Fodere, Med. Leg. Sec.  205.