The distance of the _left_ ear _e_, in the right-eye picture, from the nose _n_, will be _ne_ = N′_n_ + N′E′ - E′_e_. In order to simplify the diagram we have made the original, or left-eye picture, a front view, in which the nose is in the middle of the face, and the line joining the ears parallel to the plane of the picture. When the position of the nose and the ears has been thus approximately obtained, the artist may, in like manner, determine the place of the pupils of the two eyes, the point of the chin, the summit of the eyebrows, the prominence of the lips, and the junction of the nose with the teeth, by assuming, under the guidance of the original picture, the distance of these different parts from the plane of projection. In the same way other leading points in the figure and drapery may be found, and if these points are determined with tolerable accuracy the artist will be able to draw the features in their new place with such correctness as to give a good result in the stereoscope. In drawing the right-eye picture the artist will, of course, employ as the groundwork of it a faint photographic impression of the original, or left-eye picture, and he may, perhaps, derive some advantage from placing the original, when before the camera, at such an inclination to the axis of the lens as will produce the same diminution in the horizontal distance between any two points in the head, at a mean distance between N and N′, as projected upon the plane AB. The line N′E‴, for example, which in the left-eye photograph is a representation of the cheek NE″, is reduced, in the right-eye photograph, to _ne′_, and, therefore, if the photograph on AB, as seen by the right eye, were placed so obliquely to the axis of the lens that N′_e_ was reduced to _ne′_, the copy obtained in the camera would have an approximate resemblance to the right-eye picture required, and might be a better groundwork for the right-eye picture than an accurate copy of the photograph on AB, taken when it is perpendicular to the axis of the lens. In preparing the right-eye picture, the artist, in place of using paint, might use very dilute solutions of aceto-nitrate of silver, beginning with the faintest tint, and darkening these with light till he obtained the desired effect, and, when necessary, diminishing the shades with solutions of the hypo-sulphite of soda. When the picture is finished, and found satisfactory, after examining its relief in the stereoscope, a negative picture of it should be obtained in the camera, and positive copies taken, to form, with the original photographs, the pair of binocular portraits required. CHAPTER XVI. ON CERTAIN FALLACIES OF SIGHT IN THE VISION OF SOLID BODIES.