Moving from paintings to physical objects, let us ask ourselves the question: why, in fact, things seem to us bodily, and not flat? On the retina of our eyes the image is in fact flat. How the way that objects appear to us not as a flat picture, and the bodies of three dimensions?
There are several reasons. Firstly, the different degree of illumination of parts of objects allows us to judge their form. Secondly, plays the role of the tension that we feel when we adjust eyes to a clearer perception of the various remote parts of the bodily subject: all parts are flat pictures removed from the eyes equally, meanwhile, as part of a feature located at varying distances, and to clearly see them, the eyes have not the same “tune”. But the greatest service is provided by us that here the images in each eye from one and the same subject, is not the same. This is easily seen if you look at some near object shuting first right, then left eye. The right and left eyes see things is not the same; each draws a different picture, and it is that difference that is interpreted by our consciousness gives us the impression of relief:
Glass cube with spots, consider the left and right eye.
It seems your finger left and right eye, if you hold your hand close to the face.
Now imagine two drawings of the same subject: the first depicts the subject, as it seems the left eye, the second is right. If you look at these images so that each eye saw only own the picture, instead of two flat paintings we see one convex, raised the subject, even more prominent than physical objects visible to one eye. Consider these paired images using a special device - a stereoscope. The merge of both images was achieved in the old stereoscopes with the help of mirrors, and at the latest - with convex glass prisms: they refract the rays so that when a thought of their continuation of both images (slightly increased due to the convexity of the prisms) cover one another. The idea of the stereoscope, as you can see, very simple, but amazing action achievable means so simple.
Most readers, no doubt, happened to see a stereoscopic pictures of various scenes and landscapes. Other, may be viewed in the stereoscope and drawings of shapes made to facilitate the study of stereometria. In the future we won't talk about these more or less well-known applications of the stereoscope, and will focus only on those with which many readers are probably unfamiliar.