Lustrum censors in the Campus Martius, after the taking of the census was over. As this purification took place only once in five years, the word lustrum was also used to designate the time between two lustra. The first lustrum was performed in 566 BC by King Servius, after he had completed his census, and afterwards it is said to have taken place regularly every five years after the census was over. In the earliest period of the republic, the business of the census and the solemnities of the lustrum were performed by the consuls. The first censors were appointed in
How many angels can dance on the head of a pin? about how many could perch at the same time on the head of a pin"". Dorothy L. Sayers argued that the question was "simply a debating exercise" and that the answer "usually adjudged correct" was stated as, "Angels are pure intelligences, not material, but limited, so that they have location in space, but not extension." Sayers compares the question to that of how many people's thoughts can be concentrated upon a particular pin at the same time. She concludes that infinitely many angels can be located on the head of a pin, since they do not occupy any space there: