The solemnity and beauty of the interior are due in large measure to the devout treatment and artistic merit of the stained glass in the windows of the apse and aisles. Except in the chancel, there has been no attempt to secure uniformity of method or sequence of theme.
The windows were treated at various dates by various artists. The only aim was to obtain the deepest devotional feeling and the highest aesthetic value that could be given by the ecclesiastical art of that day. The Gothic character of the edifice of course controlled the general treatment of the windows.
The six great lancets of the apse were treated subsequent to 1885. They are the workmanship of Messrs. Clayton & Bell of London. Each lancet, bisected by its mullion, displays in its upper section two life-size figures of angels bearing musical instruments. The light streams into the chancel, solemnized by their sweet majestic faces and the rich vestments which fall in stately lines about their forms. The angels of the middle lancet above the altar stand with hands folded in prayer. Beneath this chorus of angels each lancet displays a significant scene in the life of St. Peter. The serenity and joy of the angelic chorus are in sharp contrast to the human struggle depicted in the scenes from the history of the great Apostle.
The aisle windows are large and dignified openings, with two stone mullions and elaborate tracery in each window. The three spacious compartments surmounted by a cusped circle give ample exposures for the decorative treatment of the glass. Like the chancel lancets, all the aisle windows have been treated by eminent English artists, who have adopted a key of color suited to our vivid American sunlight.