It should be mentioned, that no optical instrument of any kind is absolutely necessary to obtain a stereoscopic effect from two suitable drawings or photographs, as it is quite possible by a little management of the eyes to cause the two images to combine with each other. Referring again to fig. 67, it will be perceived that the two figures of the dice are about an inch and a half from each other. Holding the book at about ten inches from the eye, they are viewed by squinting strongly until the _right_ eye looks at the _left_ die, and the _left_ eye at the right. This may be also done by converging the eyes on a point beyond the centre of the figure, which may be easily done by looking at a point midway between the two. In both cases the images at first appear doubled, and we see four dice, but a little practice will soon enable you to cause the two inside images to coalesce, and so give the effect of relief. It is true that even then three images are seen, but the eye soon grows accustomed to neglect them altogether. This habit is a very pleasant acquirement for the London _flâneur_, who can thus see in perfection the numberless stereoscopic views now shown in our shop-windows without the intervention of an instrument of any kind. The method of photographing subjects for the stereoscope is very simple, and consists in taking two views of the object to be depicted, from two different points. According to the distance of these points from each other, so will the resulting pictures appear in greater or less relief. This is readily seen in some stereoscopic portraits which have been taken at a large angle, and consequently show such increased relief as to produce distortion. Theoretically, the interval of the two points of view ought to be two inches and a half, that being the average distance between the two eyes; but in practice it is better to increase it in the case of portraits or other near objects to about twelve inches, and in that of views to even several feet. Brewster’s original rule for taking stereoscopic photographs, was to place the cameras one foot apart for every twenty-five feet of distance. The beautiful stereoscopic pictures of the moon photographed by Mr. Warren de la Rue were taken at more than 1,000 miles’ distance, in order to obtain the necessary relief. The principle of the stereoscope has received many useful applications in the way of book illustrations, art teaching, and anatomical demonstration, and has thus gained a position among philosophical instruments that it did not at first possess.