Isaac Newton, in a letter to John Locke, recalls the close call he had previously faced when, staring for hours alternately at the sun and then at the darkness to observe the images produced on the retina, he had been temporarily blinded. He had to shut himself in a darkened room for three days. Another time, he had placed a splint between his eye and bone to see the colored circles produced by the altered curvature of the retina.
