This is called the "Telephoto Attachment," and was employed in making the telephoto illustrations here shown. The tube is 3¼ inches long and 1½ inch in diameter. When this lens is attached to the ordinary lens the time of exposure is necessarily increased, because only a few of the rays of light which diverge from the positive or ordinary lens pass through the negative lens to the plate. This is a serious drawback, for it not only debars one from using it upon moving subjects, but also increases the liability of the image to be blurred by vibrations of the camera. In order to obtain the best results the camera must be very rigid. Most of the cameras and tripods of to-day are too light and unstable for telephotography. The method of using the telephoto attachment is very simple, but requires very great care, particularly in the matter of focussing. Suppose that an exposure has been made in the ordinary way upon a certain object; the lens is then removed from the camera front and screwed into the tube of the telephoto attachment, forming a small telescope; the whole combination is then put back on the camera as if it were the ordinary lens. Upon the ground glass or focussing screen will be seen an enlarged image which may be made sharp or distinct by adjusting the focus by means of the rack and pinion movement on the telephoto tube, just as a field glass is adjusted to suit the eyes of the observer. If greater amplification be desired it is obtained by moving the front of the camera, holding the lenses farther from the ground-glass and then readjusting the focus as before. It will be seen from this that the attachment forms a lens of variable focus, changeable at the pleasure of the operator within the limits of the camera. [Illustration: Roof and Dome, Milan Cathedral—Telephoto Lens, from Same Corner of Piazza.] Some of the attachments on the market require a camera with a very long bellows, because the difference between the foci of the negative and positive lenses is not great enough to give ample power unless the combination is several feet from the plate. With my own attachment, eight inches from the plate the image is equal to that formed by an ordinary lens of twenty-four inches focus; while at twenty-four inches from the plate it is equivalent to that of a lens of sixty-four inches focus. The camera used in making the accompanying illustrations takes a plate measuring four by five inches, and the bed allows an extension of twenty-four; and when closed for transportation the box measures seven by seven by six-and-a-half inches. * * * * *