Sentence:  Thus, while at times before our eye / The clouds about the present part, / And, smiling through them, round us lie / Soft hues of Memory's evening sky – / The <WSD>Indian summer</WSD> of the heart, / In secret sympathies of mind, / In founts of feeling which retain / Their pure fresh flow, we yet may find / Our early dreams not wholly vain! 
Meaning of the word marked with <WSD> tags:  The late autumn of life; a late flowering of activity before old age. (from early 19th c.)