It is very desirable that the operator should understand the arrangement of the lenses in the tube; it not unfrequently happens, that in taking out the "glasses" to clean them, he does not return them to their proper places, and the result is that his "camera is spoiled." A couple of illustrations and a few remarks will be sufficient to enable any one to replace the lenses in them properly. Fig. 13 represents the tube for holding the lens, and Fig. 14 shows their arrangement. It will be seen that the two back lenses have a small space between them; this separation is kept by a small tube or ring of the same circumference as the lens. The two front lenses are nearest together. It will be observed that the two thick lenses are towards each other; these are made of _flint glass_ containing much oxide of lead. The other two are double convex, and are made of _crown glass_. By noting the fact that the two cemented lenses go in the front of the tube, the glass having the thickest edge goes inside, and that the _thickest lens_ of the other two goes in first, from the back of the tube, it will not be easy for the operator to make a mistake in returning the "glasses." "I will remark that a diaphragm diminishes both chromatic and spherical aberration, by cutting off the outside portion of the lens. It lessens the brilliancy of the image, but improves the distinctness by preventing various rays from interfering with and confusing each other; it also causes a variety of objects at different distances to be in focus at the same time." [Illustration: Fig. 15.] [Sidenote: CAMERA BOXES.] The tube containing the lenses is to be mounted on a box (camera-box) as in Fig. 15. For this purpose there are several patterns of boxes, from among which I have made two selections of the most approved, and represent them by cuts, Figs. 16, 17, 18. [Illustration: Fig. 16.] [Illustration: Fig. 17.] Figs. 16 and 17 represent a bellows-box which is probably more in use than all the other patterns together. They serve both for copying and taking portraits from life. A is the base; B is the back and _sliding_-box; C, bellows, which admits of extension or contraction; D is the opening to receive the carriage A, Fig. 17; E is a thumb screw to hold the sliding-box at any required distance. Fig. 17 represents the plate-holder and ground glass frame. A, carriage to pass through D, Fig. 16; B, frame for ground-glass, which may be turned in a horizontal or perpendicular position; C, a movable plate-holder held in place by means of springs; D, reducing holder, with bottom and plate to hold the glass plate: any size of reducing frame can be put in frame C; E E, spring bottom to keep frame D in place; F, slide; G, thumb-screw, when the carriage is to be put in or taken out of the box, Fig 16; H H, spring bottom to hold B in place.