The Optical Lantern and Cinematograph Journal (Nov 1904-Oct 1905)

THE OPTICAL LANTERN AND CINEMATOGRAPH JOURNAL. 173 Paper read at the Optical Convention. Friday, Jiun 2nd. By C. W. S. Crawley. THE human eyes, faulty as they may be in many ways, are in one respect an instrument of extreme precision. As a means of instantly detecting minute differences of angle, they have a delicacy which is but seldom appreciated. Helmholtz has investigated the matter, and in his Physiological Optics describes experiments which show that the eyes are capable of detecting a difference between two angles of as little as one minute. This power of discrimination it is that enables us to judge distance by stereoscopic vision, not, indeed, to judge distance absolutely, but to say which of two objects is the nearer. To take a concrete example. Let us place two rods or other suitable objects, the one at the distance of 2 metres, and the other 2 centimetres further. Let them be nearly in the same line of sight, be perfectly evenly lighted, and have all surroundings cut off, so that there is no means of judging which is the further except the opinion that we form by looking at them. If we do so with one eye only, we could not say which was the nearer, nor could we even if they were much further apart. With two eyes, however, no one with ordinary vision would have any doubt. One rod " looks " nearer than the other, though why it does we do not and cannot consciously realise. This power of judging distance is common to all; it has always been advantageous to every member of the human race to judge distance, and to do so continually the whole time that he is awake. Evolution has consequently had its fullest opportunities, and has seized them with marvellous results. How are we able to judge that one of these rods is nearer than the other ? Surely by the fact that, when we look at it, we have to converge our eyes a little more than when we look at the other. The proximate physical fact that we unconsciously judge by is an appreciation of the comparative muscular efforts to produce the convergence of the eyes on objects at different distances. Taking the eye-distance as 65 nun., which is about the normal, the angle eye — rod — eye at 2 m. is roughly \" 57', while at 2 m. 20 cm. it is roughly \" 56'. The difference ol convergence of the eyes on the two rods is consequently only about 1 minute, and probably 98 per cent, of the population would have no hesitation about which is the nearer, and the remaining 2 per cent, would be found not to have stereoscopic vision at all. But with all reverence for a great name, Helmholtz put the limit far too high. From numerous tests in all sorts of conditions and ages of men and some women, as will be seen in Table I., we find that only in two cases was the angle even half a minute. The general angle appears to be about 10", and really good men can appreciate with certainty a difference of 2 to 3 seconds. In repeating the experiments there are one or two points that must be attended to, to ensure that the distance is judged only by stereoscopic faculty. The lighting must be even and regular, and should be exactly behind the observer, otherwise shadows may be of great help, and far too good results unintentionally obtained. The background should be fairly uniform and preferably, at least, as far off again as the rods. Ordinary wax matches make excellent rods. The holders, slides, and all surroundings must be cut off by a screen. The eyes should he on a level with the tops of the matches.