The camera is based on the same optical principle as our eye. Everything projected onto its ground-glass depends on the distance between the lens and the object. The camera gives a perspective, which we would get with one eye. Now replace our eye to the lens. So, if you want to obtain from a photograph the same visual impression that the photographed object produced, we must, firstly, look at the photograph with one eye only, and, secondly, hold it at the proper distance away. When you look at a photograph with both eyes the picture you get is flat and not three-dimensional. This is the fault of our own vision. When we look at something solid the image it causes on the retina of either eye is not the same. Our brain blends the two different images into one; this is the basic principle of the stereoscope. On the other hand, if we are looking at something that flat-a wall, for instance-both eyes get an identical sensory picture telling our brain that the object we are looking at is really flat. Now you should realize the mistake we make when we look at a photograph with both eyes. In this manner we compel ourselves to believe that the picture we have before us is flat. When we look with both eyes at a photograph which is really untended only for one eye, we prevent ourselves from seeing the picture that the photograph really shows, and thus destroy the illusion which the camera produces with such perfection.