For the 1959 Prospectus, the portrait of Emperor Taizong, on the previous year’s cover, was replaced by a picture of  Queen Victoria  Prince Albert dancing the polka. The original painting was chromolithographed by John Brandard in the 1840s, and has been in the National Portrait Gallery, London, UK since 1902. 
The polka is a Bohemian dance that originated in Central Europe in the 19th Century. The dance suggests a young lady dancing in free spirit in the street which inspired a passer-by who turned it into proper dancing steps. It then became hugely popular all over Europe. 
The picture itself referred to a literature course in the prospectus entitled, The Victorian Age and was another example of a course-related cover. In later years  the practice of linking a cover illustration to a course in the prospectus was dropped.