If parallel rays of light fall upon a double-convex lens, D D, Fig. 8, they will be refracted (excepting such as pass directly through the centre) to a point termed the principal focus. The lines A B C represent parallel rays which pass through the lens D D, and meet at F; this point being the principal focus, its distance from the lens is called the _focal length_. Those rays of light which are traversing a _parallel_ course, when they enter the lens are brought to a focus nearer the lens than others. Hence the difficulty the operator sometimes experiences by not being able to "obtain a focus," when he wishes to secure a picture of some very distant objects; he does not get his _ground glass near enough to the lenses_. Again, the rays from an object near by may be termed diverging rays. This will be better comprehended by reference to Fig. 9, where it will be seen that the dotted lines, representing parallel rays meet nearer the lenses than those from the point A. The closer the object is to the lenses, the greater will be the divergence. This rule is applicable to copying, Did we wish to copy a 1/6 size daguerreotype on a 1/16 size plate, we would 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. These remarks have been introduced here as being important for those who may not understand the principles of enlarging or reducing pictures in copying. [Sidenote: LENSES.] 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 wood-cut:-- [Illustration: Fig. 10.]