The usual explanation of the appearance of the optical image on the back inside wall of such a camera is that light-rays, emanating from every point outside, cross each other in the aperture of the camera and so - again point by point - create the inverted image. No such explanation, clearly, is open to us. For the world of external objects is a whole, and so is its image appearing in the camera. Equally, the light entering the camera is not a sum of single rays. Pure observation leads to the following description of the optical process.