We have already seen, as shewn in Fig. 22, that when we fix the binocular centre, that is, converge the optic axes on a point beyond the dissimilar pictures, so as to unite them, they rise into relief as perfectly as when the binocular centre, as shewn in Fig. 18, is fixed between the pictures used and the eye. In like manner we may unite similar pictures, but, owing to the opacity of the wall and the floor, we cannot accomplish this with paper-hangings and carpets. The experiment, however, may be made with great effect by looking through transparent patterns cut out of paper or metal, such as those in zinc which are used for larders and other purposes. Particular kinds of trellis-work, and windows with small squares or rhombs of glass, may also be used, and, what is still better, a screen might be prepared, by cutting out the small figures from one or more pieces of paper-hangings. The readiest means, however, of making the experiment, is to use the cane bottom of a chair, which often exhibits a succession of octagons with small luminous spaces between them. To do this, place the back of the chair upon a table, the height of the eye either when sitting or standing, so that the cane bottom with its luminous pattern may have a vertical position, as shewn in Fig. 25, where MN is the real bottom of the chair with its openings, which generally vary from half an inch to three-fourths. Supposing the distance to be half an inch, and the eyes, L, R, of the observer 12 inches distant from MN, let L_ad_, L_be_ be lines drawn through the centres of two of the open spaces _a_, _b_, and R_bd_, R_ce_ lines drawn through the centres of _b_ and _c_, and meeting L_ad_, L_be_ at _d_ and _e, d_ being the binocular centre to which the optic axes converge when we look at it through _a_ and _b_, and _c_ the binocular centre when we look at it through _b_ and _c_. Now, the right eye, R, sees the opening _b_ at _d_, and the left eye sees the opening _a_ at _d_, so that the image at _d_ of the opening consists of the similar images of _a_ and _b_ united, and so on with all the rest; so that the observer at L, R no longer sees the real pattern MN, but an image of it suspended at _mn_, three inches behind MN. If the observer now approaches MN, the image _mn_ will approach to him, and if he recedes, _mn_ will recede also, being 1½ inches behind MN when the observer is _six_ inches before it, and _twelve_ inches behind MN when the observer is _forty-eight_ inches before it, the image _mn_ moving from _mn_ with a velocity one-fourth of that with which the observer recedes.