During the 1850s, the cost of the "Transactions" to the Society was increasing again (and would keep doing so for the rest of the century); illustrations were always the largest expense. Illustrations had been a natural and essential aspect of the scientific periodical since the later seventeenth century. Engravings (cut into metal plates) were used for detailed illustrations, particularly where realism was required; while wood-cuts (and, from the early nineteenth century, wood-engravings) were used for diagrams, as they could be easily combined with letterpress.