This rule is applicable to copying.

Did we wish to copy a 1/6 size Daguerreotype on a 1/16 size plate, we should place it in such a position to the lenses at A that the focus would be at F, where the image would be represented at about the proper size.

Now, if we should wish to copy the 1/6 size picture, and produce another of exactly the same dimensions, we have only to bring it nearer to the lenses, so that the lens D E shall be equi-distant from the picture and the focus, i.e.from A to B.The reason of this is, that the distance of the picture from the lens, in the last copy, is less than the other, and the divergence has increased, throwing, the focus further from the lens." [Illustration: Fig.

9 (amdg_9.gif)] These remarks have been introduced here as being important for those who may not understand the principles of enlarging or reducing pictures in copying.
I would remark that the points F and A, in Fig.

9, are termed "conjugate foci." If we hold a double-convex lens opposite any object, we find that an inverted image of that object will be formed on a paper held behind it.
To illustrate this more clearly, I will refer to the following woodcut: [Illustration: Fig.