<P> Two major theories on vision prevailed in classical antiquity . The first theory, the emission theory, was supported by such thinkers as Euclid and Ptolemy, who believed that sight worked by the eye emitting rays of light . The second theory, the intromission theory supported by Aristotle and his followers, had physical forms entering the eye from an object . Previous Islamic writers (such as al - Kindi) had argued essentially on Euclidean, Galenist, or Aristotelian lines . The strongest influence on the Book of Optics was from Ptolemy's Optics, while the description of the anatomy and physiology of the eye was based on Galen's account . Alhazen's achievement was to come up with a theory that successfully combined parts of the mathematical ray arguments of Euclid, the medical tradition of Galen, and the intromission theories of Aristotle . Alhazen's intromission theory followed al - Kindi (and broke with Aristotle) in asserting that "from each point of every colored body, illuminated by any light, issue light and color along every straight line that can be drawn from that point". This however left him with the problem of explaining how a coherent image was formed from many independent sources of radiation; in particular, every point of an object would send rays to every point on the eye . What Alhazen needed was for each point on an object to correspond to one point only on the eye . He attempted to resolve this by asserting that the eye would only perceive perpendicular rays from the object--for any one point on the eye only saw the ray that reached it directly, without being refracted by any other part of the eye, would be perceived . He argued using a physical analogy that perpendicular rays were stronger than oblique rays; in the same way that a ball thrown directly at a board might break the board, whereas a ball thrown obliquely at the board would glance off, perpendicular rays were stronger than refracted rays, and it was only perpendicular rays which were perceived by the eye . As there was only one perpendicular ray that would enter the eye at any one point, and all these rays would converge on the centre of the eye in a cone, this allowed him to resolve the problem of each point on an object sending many rays to the eye; if only the perpendicular ray mattered, then he had a one - to - one correspondence and the confusion could be resolved . He later asserted (in book seven of the Optics) that other rays would be refracted through the eye and perceived as if perpendicular . </P> <P> His arguments regarding perpendicular rays do not clearly explain why only perpendicular rays were perceived; why would the weaker oblique rays not be perceived more weakly? His later argument that refracted rays would be perceived as if perpendicular does not seem persuasive . However, despite its weaknesses, no other theory of the time was so comprehensive, and it was enormously influential, particularly in Western Europe: Directly or indirectly, his De Aspectibus (Book of Optics) inspired much activity in optics between the 13th and 17th centuries . Kepler's later theory of the retinal image (which resolved the problem of the correspondence of points on an object and points in the eye) built directly on the conceptual framework of Alhazen . </P> <P> Alhazen showed through experiment that light travels in straight lines, and carried out various experiments with lenses, mirrors, refraction, and reflection . His analyses of reflection and refraction considered the vertical and horizontal components of light rays separately . </P> <P> The camera obscura was known to the ancient Chinese, and was described by the Han Chinese polymathic genius Shen Kuo in his scientific book Dream Pool Essays, published in the year 1088 C.E. . Aristotle had discussed the basic principle behind it in his Problems, however Alhazen's work also contained the first clear description, outside of China, of camera obscura in the areas of the middle east, Europe, Africa and India . and early analysis of the device . </P>

Who discovered that light travels in straight lines
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