Rare chart of west Cornwall from The Lizard to Land End, published in Blaeu's rare pilot 'Zeespiegel Inhoudende Een...'.

The 'Zeespeigel' was the second of Blaeu's great pilot guides: the first, 'Het Licht der Zeevaert', was published in various editions and languages between 1608 and 1630. Blaeu's copyright to this work appears to have run out in 1618, and from 1620 Johannes Janssonius was publishing his own counterfeit versions. Blaeu responded to this threat from his rival by publishing the present work in 1623. The new work covered much the same geographical area, i.e. the northern, eastern (the Netherlands to the White Sea) and western (the Netherlands to the Barbary Coast) navigations, however on a much larger scale and with more than twice the number of charts (111 compared to the Zeevaert's 42).

Although the new pilot proved hugely successful and would continue to be published for the next 30 years, its practical application aboard ship accounts for its extreme rarity today.