(2) A spondee is allowable in any position; the limit is four out of five, with either the fourth or fifth foot remaining iambic.
(3) A pyrrhic may occur in any position; the limit is three out of five, with the other feet preferably spondees.
(4) The limit for trisyllabic substitution is three out of five.
(Chapters on English Metre, chap. V.)
Professor Corson discusses the æsthetic effect of these changes from the typical metre: "The true metrical artist ... never indulges in variety for variety's sake.... All metrical effects are to a great extent relative—and relativity of effect depends, of course, upon having a standard in the mind or feelings.... Now the more closely the poet adheres to his standard—to the even tenor (modulus) of his verse—so long as there is no logical nor æsthetic motive for departing from it, the more effective do his departures become when they are sufficiently motived. All non-significant departures weaken the significant ones.... The normal tenor of the verse is presumed to represent the normal tenor of the feeling which produces it. And departures from that normal tenor represent, or should represent, variations in the normal tenor of the feeling. Outside of the general law ... of the slurring or suppression of extra light syllables, which do not go for anything in the expression, an exceptional foot must result in emphasis, whether intended or not, either logical or emotional.... A great poet is presumed to have metrical skill; and where ripples occur in the stream of his verse, they will generally be found to justify themselves as organic; i.e. they are a part of the expression."
(Primer of English Verse, pp. 48-50.)
On the æsthetic symbolism of various metrical movements, see G. L. Raymond's Poetry as a Representative Art, pp. 113 ff.
FOOTNOTES:
[6] The names of the several kinds of feet are of course borrowed from classical prosody, where they are used to mark feet made up not of accented and unaccented, but of long and short syllables. The different significance of the terms as applied to the verse of different languages has given rise to some confusion, and it is proposed by some to abandon the classical terms; their use, however, seems to be too well established in English to permit of change. Some would even abandon the attempt to measure English verse by feet, contending that its rhythm is too free to admit of any such measuring process; thus, see Mr. J. M. Robertson, in the Appendix to New Essays toward a Critical Method, and Mr. J. A. Symonds in his Blank Verse. See also in Mr. Robert Bridges's Milton's Prosody, Appendix G "On the Use of Greek Terminology in English Prosody." (1901 ed. p. 77.)
[7] On the historical problem of the distinction between elided and genuinely hypermetrical syllables in the French and English decasyllabic verse, see Motheré: Les théories du vers héroique anglais et ses relations avec la versification française (Havre, 1886).