The astronomer Johannes Kepler was the first scientist to explain that the eye works by focusing an image of the outside world onto the retina. However, we can see floaters just in front of the retina and white blood cells moving in the capillaries on the retina because of the shadows they cast and the light they let through. No focusing of an image is involved. Can you think of any other things and ways we might see that also do not rely on the focusing of an image on the retina?