 The popularity of Della Porta's books helped spread knowledge of the camera obscura.In his 1567 work "La Pratica della Perspettiva" Venetian nobleman Daniele Barbaro (1513-1570) described using a camera obscura with a biconvex lens as a drawing aid and points out that the picture is more vivid if the lens is covered as much as to leave a circumference in the middle.In his influential and meticulously annotated Latin edition of the works of Ibn al-Haytham and Witelo "Opticae thesauru" (1572) German mathematician Friedrich Risner proposed a portable camera obscura drawing aid; a lightweight wooden hut with lenses in each of its four walls that would project images of the surroundings on a paper cube in the middle.