In full confidence of this unalterable truth, I now (quod felix faustumque sit) lay the first stone of the Temple of Peace.
Over and above the enclosing brackets, a parenthesis causes no change in the punctuation of the sentence that contains it; in other words, if we were to omit the parenthesis, no change ought to be necessary in the punctuation of the rest of the sentence. The comma is inserted after the parenthesis in the first example, because the comma would be needed even if there were no parenthesis.
In the second example, there would be no comma before "lay," if there were no parenthesis; accordingly the comma is not to be inserted merely because there is a parenthesis. A parenthesis is sufficiently marked off by brackets.
Observe also that the comma in the first example is placed after, not before, the parenthesis. The reason for this is that the parenthesis belongs to the first part of the sentence, not to the second.
LI. A complete sentence occurring parenthetically in a paragraph is sometimes placed within brackets.
Godfrey knew all this, and felt it with the greater force because he had constantly suffered annoyance from witnessing his father's sudden fits of unrelentingness, for which his own habitual irresolution deprived him of all sympathy. (He was not critical on the faulty indulgence which preceded these fits; that seemed to him natural enough.) Still there was just the chance, Godfrey thought, that his father's pride might see this marriage in a light that would induce him to hush it up, rather than turn his son out and make the family the talk of the country for ten miles round.
Note that the full stop should be placed inside, not outside, the brackets.
LII. Where, in quoting a passage, we throw in parenthetically something of our own, we may use square brackets.
Compare the following account of Lord Palmerston: "I have heard him [Lord Palmerston] say that he occasionally found that they [foreign ministers] had been deceived by the open manner in which he told them the truth."
"The Leviathan of Hobbes, a work now-a-days but little known [and not better known now than in Bentham's time], and detested through prejudice, and at second-hand, as a defence of despotism, is an attempt to base all political society upon a pretended contract between the people and the sovereign."—Principles of Legislation.