Between the pictures and the purely optical part of the stereoscope, there are other relations of very considerable importance. The exclusion of all external objects or sources of light, excepting that which illuminates the pictures, is a point of essential importance, though its advantages have never been appreciated. The spectacle stereoscope held in the hand, the reflecting stereoscope, and the open lenticular stereoscope, are all, in this respect, defective. The binocular pictures must be placed in a dark box, in order to produce their full effect; and it would be a great improvement on the lenticular stereoscope, if, on the left and right side of each eye-tube, a piece of brass were to be placed, so as to prevent any light from entering the left angle of the left eye, and the right angle of the right eye.[57] The eyes, thus protected from the action of all external light, and seeing nothing but the picture, will see it with a distinctness and brilliancy which could not otherwise be obtained. [57] When any external light falls upon the eye, its picture is reflected back from the metallic surface of the Daguerreotype, and a negative picture of the part of the Daguerreotype opposite each eye is mixed with the positive picture of the same part. The proper illumination of the picture, when seen by reflected light, is also a point of essential importance. The method universally adopted in the lenticular stereoscope is not good, and is not the one which I found to be the best, and which I employed in the first-constructed instruments. The light which falls upon the picture is prevented from reaching the observer only by its being incident at an angle greater or less than the angle of reflexion which would carry it to his eyes. A portion of the scattered light, however, does reach the eye, and in Daguerreotypes especially, when any part of the surface is injured, the injury, or any other imperfection in the plate, is more distinctly seen. The illumination should be lateral, either by a different form of window in the front, or by openings on the two sides, or by both these methods. When the lenticular stereoscope is thus fitted up, and the pictures in this manner illuminated, the difference of effect is equally great as it is between a picture as commonly seen, and the same picture exhibited as a panorama or a diorama, in which no light reaches the eyes but that which radiates from the painting itself, the reflexion from the varnish being removed by oblique or lateral illumination.