No Beam From the Eye

Light Through the Ages: Ancient Greece to Maxwell is a very fascinating article written by J J O'Connor and E F Robertson in 2002 that takes us through a journey of how our understanding of light has evolved over the last few millennia. Here is an excerpt from the article:

The biggest breakthrough in ancient times was made by al-Haytham around 1000 AD. He argued that sight is due only to light entering the eye from an outside source and there is no beam from the eye itself.

The article then goes on to say:

He gave a number of arguments to support this claim, the most persuasive being the camera obscura, or pinhole camera. Here light passes through a pinhole shining on a screen where an inverted image is observed. Anyone visiting Edinburgh in Scotland should go to see the camera obscura there near the top of the Royal Mile and marvel at just how effective the camera obscura is in this enjoyable tourist attraction.

By the way, there is a camera obscura in the London too at Royal Observatory, Greenwich which offers pretty good view of the surroundings that includes Greenwich Park, the University of Greenwich, the River Thames, etc. Here is a picture I took the last time I was there: Camera Obscura, Royal Observatory, Greenwich.