Now, suppose a second lens be placed close to the first, (Fig. 94.) having for its principal focal length or
In order to find the distance of the focus after the second refraction, we must consider and as representing and in the formula, so that
or
or
And in like manner, if there be any number of lenses acting together, we shall have
or
so that their joint effect is the same as that of a single lens, having, for its principal focal length unity divided by
or
95. Mr. Herschel calls the reciprocal quantity the power of a lens, and enounces the last result thus:
"The power of any system of lenses is the sum of the powers of the component lenses."
Of course, regard must be had to the signs: the power of a concave-lens must be considered as positive, that of a convex one, negative.
96. The same method by which we found the focal length of a lens may be easily applied to any number of surfaces, having a common axis.