Shen Kuo experimented with the pinhole camera and burning mirror as the ancient Chinese Mohists had done in the 4th century BC. Mozi first described the concept of camera obscura 2000 years earlier during the Warring States period. An Iraqi Muslim scientist Ibn al-Haytham (965–1039) further experimented with camera obscura. Then Shen Kuo was the first to attribute geometrical and quantitative properties to the camera obscura. Using a fitting metaphor, Shen compared optical image inversion to an oarlock and waisted drum. He also discussed focal points and noted that the image in a concave mirror is inverted. Shen, who never asserted that he was the first to experiment with camera obscura, hints in his writing that camera obscura was dealt with in the "Miscellaneous Morsels from Youyang" written by Duan Chengshi (d. 863) during the Tang Dynasty (618–907), in regard to the inverted image of a Chinese pagoda by a seashore.