refraction "is basically in accord with modern scientific principles." In Europe, Roger Bacon (1214–1294) was the first to suggest that the colors of the rainbow were caused by the reflection and refraction of sunlight through rain drops. Shen hypothesized that rays of sunlight refract before reaching the surface of the earth, hence people on earth observing the sun are not viewing it in its exact position, in other words, the altitude of the apparent sun is higher than the actual altitude of the sun. Dong writes that "at the time, this discovery was remarkably original." Ibn al-Haytham, in his Book of Optics (1021), also discussed atmospheric refraction in regard to twilight. Astronomy and instruments Being the head official for the Bureau of Astronomy, Shen Kuo was an avid scholar of medieval astronomy, and improved the designs of several astronomical instruments. Shen is credited with making improved designs of the gnomon, armillary sphere, and clepsydra clock. For the clepsydra he designed a new overflow-tank type, and argued for a more efficient higher-order interpolation instead of linear interpolation in calibrating the measure of time. Improving the 5th century model of the astronomical sighting tube, Shen Kuo widened its diameter so that the new calibration could observe the pole star indefinitely. This came about due to the position of the pole star shifting in position since the time of Zu Geng in the 5th century, hence Shen Kuo diligently observed the course of the pole star for three months, plotting the data of its course and coming to the conclusion that it had shifted slightly over three degrees. Apparently this astronomical finding had an impact upon the intellectual community in China at the time. Even Shen's political rival and contemporary astronomer Su Song featured Shen's corrected position of the pole star (halfway between Tian shu, at −350 degrees, and the current Polaris) in the fourth star map of his celestial atlas. The astronomical phenomena of the solar eclipse and lunar eclipse had been observed in the 4th century BC by astronomers Gan De and Shi Shen; the latter gave instructions on predicting the eclipses based on the relative position of the Moon to the Sun. The philosopher Wang Chong argued against the 'radiating influence' theory of Jing Fang's writing in the 1st century BC and that of Zhang Heng (78–139); the latter two correctly hypothesized that the brightness of the Moon was merely