Procure a printing frame, such as photographers use; lay the tracing, face down, upon the glass, upon which place the sensitive paper, prepared side down, then several thicknesses of cotton flannel for a pad to equalize the pressure, and cause the sensitive paper and tracing to lay in close contact, and then close in the back. If, on turning the frame over, any wrinkles appear, that side of the hinged back may be opened and a piece of paper laid in just above the spot, when all will come smooth on closing the frame, (this should be done in a dimly lighted room), then expose to direct sunlight, care being taken that the whole frame comes under the light, without shadows; let the exposure be from five to ten minutes, according to the brightness of the day. Remove again to darkened room, examine by opening one of the hinged backs; if the lines have slightly turned in color, it has been highly exposed; it can be removed and washed in clear water, with two or three changes, then hang up to dry. You will have an exact copy of the original, with white lines on a blue ground, at a cost of about one tenth that of tracing, with absolutely no error. The paper must be kept in perfect darkness until used.