Colonnades V

The colonnades lanterns in St Peter's Square, Vatican city, Rome, Italy. Each one is inscribed with "San Pietro in Vat. Anno MDCCCLV" translated meaning 'St Peter in Vatican year 1855'. Why that date I'm unsure and I couldn't find any real answer on the internet, in 1855 Pope Pius IX reigned, he was known as the "friend of light" to protestants, it was also the year of a concordat between Austria and the Papacy, whether any of these are relevant I don't know, maybe it's just the year the lamps were installed.