The “convertible” lenses of various makers are also separable, and where the rear and front lenses are of different foci three focal lengths are obtainable. These lenses are of the highest grade, and consequently expensive. In a bright light, or where great speed is not required, they do not seem to be as superior to the trade lens as the much higher price would lead one to expect. But in dull days, or in the shadow, or where extremely rapid exposures are necessary, their superior qualities become evident. My experience with these convertible lenses has been limited to the Zeiss Anastigmat, Series VII _a_, of which I am now using a No. 10 with a combined focus of eight inches, the front and rear lenses both having a focal length of fourteen inches. This combination is preferred to one in which the component lenses are of different foci, because of the greater speed of the two when combined, and furthermore, because, being of the same focus, they could, if occasion arose, be used in a twin-lens box. The speed of the combination is registered at F. 6.3; that of the single lenses at 12.5. With the former the most rapid exposures can be made successfully, while the latter are sufficiently fast to permit of ordinary instantaneous work. This lens is stated to cover a 5 × 8½ plate, and when in use on a 4 × 5 camera gives a high degree of illumination and perfect definition. The telephoto lens may be employed in certain kinds of bird photography with not unsatisfactory results. Its disadvantages are lack of speed, an exposure of at least one half a second to a second being required at F. 8 in bright sunlight, the necessity of extreme care in focusing, and of absolute rigidity of the camera at the time of making the exposure. In short, the telephotographer needs more time, both before and after pressing his bulb, than the bird photographer is often accorded. However, with such subjects as nests high in trees or on cliffs, Herons and other shore-inhabiting birds, Ducks on the water or Hawks perched in leafless trees, the telephoto will be found serviceable. Negatives are frequently secured in which the figure of the bird, while small, is sharp, when, by enlargement, a desirable picture can be made of what in the original was too small to be easily distinguishable. An increase in size of two diameters is possible from any fairly sharp negative, but if the object be in perfect focus an increase of four diameters may be made. These enlargements may be made with an enlarging camera or with the aid of a Nehring enlarging lens, which is placed between the front and back lenses of the view lens, when, with the ordinary long-focus camera, a magnification of about four diameters may be obtained, the image being thrown on to a piece of bromide paper in the plate holder.