Last week I went to see an optometrist. As he examined my eye with a strong light, I was amazed that I saw an image of the blood vessels on the retina. This phenomenon is called the entoptic image. It was first described by Purkinje about 200 years ago. I read about it in “How we perceive our own retina” by Kuno Kirschfeld (published in Proc. Bio. Sci.)