Whenever anything is so far away that we cannot see it distinctly, we make use of a field-glass or telescope, which produces a magnified image of the object so that we are able to perceive what the unaided eye could not. In a similar manner the telephoto attachment enlarges the image formed by the ordinary lens in the camera. To produce on a photographic plate an image that fairly resembles what our eyes see, requires a lens of much longer focus than is generally used, and a camera that would permit the use of such a lens would be unwieldy and too cumbersome for a peripatetic photographer, and simply impossible for a mountain-climber. The telephoto lens overcomes this difficulty by producing the effect of a lens of long focus in a very compact camera. It would be interesting to know who first applied this form of lens to a camera for the purpose of photographing distant objects. In 1890, while experimenting with the lenses from an old field-glass, I discovered that a dim yet distinct image of St. Patrick's Cathedral spires was formed in my camera, although the Cathedral was eighteen blocks away. After making several exposures with this combination of lenses I became convinced that with lenses of the best possible optical construction wonderful results might be attained. Having previously purchased a telescope with a three-and-a-half inch lens of sixty inches focus (with the idea of attaching it to a long box-camera as a photographic lens for the purpose of making photographs of distant terrestrial objects, as astronomers photograph heavenly bodies). I found that the field-glass combination of lenses yielded an image nearly as large as that produced by the telescope lens, and that too with a camera only one third the length of the other. [Illustration: Milan Cathedral from Opposite Corner of Piazza.] Becoming deeply interested in this line of investigation I called upon a celebrated lens maker in London and learned that he had manufactured what he called a "Compound Telephoto Lens" consisting of a portrait lens with a small negative or concave lens adjusted at a suitable distance back of it. This instrument was too large and cumbersome for my small camera, and shortly afterward a negative lens, with a rack and pinion mounting, was manufactured of such a size that it could be attached to any fine rectilinear lens of suitable focus, although in some cases special corrections are necessary.