A Manual Of Photography

The lenticular instrument, fitted for use, is shown in figure 86: it consists of a frame of wood or metal; the two semi-lenses are fixed in brass tubes which are capable of 308 PRACTICE OF PHOTOGRAPHY. " When we thus view,” says Sir David Brewster, “ two dis- similar drawings of a solid object, as it is seen by each eye separately, we are actually looking through two prisms, which produce a second image of eaeli drawing, and when these second images unite, or coalesce, we see the solid image which they represent. But in order that the two images may coalesce, without any effort or strain on the part of the eye, it is neces- sary that the distance of the similar parts of the two drawings be equal to twice the separation produced by the prism, dor this purpose measure the distance at which the semi-lenses give the most distinct view of the drawings ; and having ascertained, bv using one eye, tlie amount of the refraction produced at that distance, or the quantity by whicli the image of one of the drawings is displaced, place the drawings at a distance equal to twice that quantity ; that is, place the drawings so that the average distance of similar parts in each is equal to twice that quantity. If this is not correctly done, the eye of the observer will correct the error by making the images coalesce without being sensible that it is malting any such effort. AV hen the dissimilar drawings are thus united, the solid will appear stand- ing, as it were, in relief, between the two plane representations.”