To procure these minute photographs, an ordinary negative must be taken, great care being necessary to obtain a negative that is perfectly clear at the edges, as well as in the centre. This operation will produce a photograph as much smaller than the original as the. power of the lens and length of focus will allow. From a print taken off the negative thus obtained, another negative still more reduced must be taken, and this operation repeated if necessary until the final photograph is obtained of the desired size. The last positive must be printed on very fine transparent paper, and all the usual operations for toning and fixing carefully performed. On arriving at its destination, the letter must be carefully unrolled and mounted on glass, then by the aid of a gas microscope attached to a powerful phantasmagoria lantern the image can be reflected on to a screen, and will be so much magnified as to be easily read And transcribed. If rapidity of copying is desired, the image on the screen may be . divided into petitions by lines, and several persons set to copy at the same time.