The best thickness for the boards is 1 in., but for cheapness 3/4 in. will do as well, yet the saving is so little that the 1-in. boards are preferable.

The dark room shown in the accompanying sketch measures 3 ft. 6 in. by 2 ft. 6 in., the height to the eaves being 6 ft.  Form the two sides shown in Fig 1, fixing the crosspieces which hold the boards together in such positions that the bottom one will act as a bearer for the floor, and the second one for the developing bench.  Both sides can be put together in this way, and both exactly alike.  Keep the ends of the crosspieces back from the edges of the boards far enough to allow the end boards to fit in against them.

One of the narrow sides can be formed in the same way, fixing the crosspieces on to correspond, and then these three pieces can be fastened together by screwing the two wide sides on the narrow one.

Lay the floor next, screwing or nailing the boards to the crosspieces, and making the last board come even with the ends of the crosspieces, not even with the boards themselves.  The single boards can then be fixed, one on each side of what will be the doorway, by screwing to the floor, and to the outside board of the sides.  At the top of the doorway, fix a narrow piece between the side boards, thus leaving a rectangular opening for the door.

The roof boards may next be put on, nailing them to each other at the ridge, and to the sides of the room at the outsides and eaves.  They should overhang at the sides and eaves about 2 in., as shown in Figs. 3 and 4.

One of the sides with the crosspieces in place will be as shown in Fig. 2 in section, all the crosspieces and bearers intersecting around the room.

The door is made of the same kind of boards held together with crosspieces, one of which is fastened so as to fit closely to the floor when the door is hinged, and act as a trap for the light.  The top crosspiece is also fastened within 1 in. of the top of the door for the same reason.

Light traps are necessary at the sides and top of the door.  That at the hinged side can be as shown at A, Fig. 5, the closing side as at B, and the top as at C in the same drawing.  These are all in section and are self-explanatory.  In hinging the door, three butt hinges should be used so as to keep the joint close.

The fittings of the room are as shown sectionally in Fig. 6, but before fixing these it is best to line the room with heavy, brown wrapping paper, as an additional safeguard against the entrance of light.

The developing bench is 18 in. wide, and in the middle an opening, 9 by 11 in., is cut, below which is fixed the sink.  It is shown in detail in Fig. 7, and should be zinc lined.

The zinc should not be cut but folded as shown in Fig. 8, so that it will fit inside the sink.  The bench at each side of the sink should be fluted (Fig. 9), so that the water will drain off into the sink.  A strip should be fixed along the back of the bench as shown in Figs. 6 and 9, and an arrangement of slats (Fig. 10), hinged to it, so as to drop on the sink as in Fig. 6, and shown to a larger scale in Fig. 11.