and of a second divergent meniscus consisting of two simple lenses-one a double concave, of flint-glass, which is next the anterior lens; the other a convergent meniscus of crown-glass

This objective, shown in section in Fig. 63, can be used with its entire aperture (about), and then acts rapidly, but the sharp image is limited in size to about half the focal length. The aperture can be decreased in diameter to, by means of imbricated plates of brass, which form the diaphragm. With the aperture of

the image is rendered sharp to an extent equal to the focal length. As the orthoscopic produces what is called pincushion distortion it is not adapted for reproducing maps, buildings, engravings, &c.

The Double Portrait Lens of Petzval consists of an achromatised meniscus, nearly plano-convex, the convex face, A, being towards the object, and of a double-convex combination, B c, formed of a divergent meniscus, B, of flint glass, situated at a certain distance from the double-convex crown-glass lens, c. It is shown in section of real size in Fig. 64, as constructed by Dallmeyer, of London.

The apertures of the diaphragm, D D', are so graduated, that their relative times of exposure are found by simple multiplication.

If the whole aperture of this objective be used, the field is much curved. With an aperture of, it covers sharply not more than of the focal plane; with an aperture of, the extent of image sharply defined becomes from to ; and with an aperture of, the sharply defined image is equal in extent to the focal length.

If the diaphragm be placed as shown in Fig. 64, this objective produces little or no distortion.

B

Fig. 64.

Mr. Dallmeyer's Triplet, shown in section of real size in Fig. 65, is composed of three achromatised meniscuses, A B, C D, E F, each formed of two simple lenses cemented together at their common surfaces. The tube in which the three meniscuses are mounted is covered with a shutter at G H, and screws on to a flange, I K, fixed to the camera. The combination E F is turned towards the object, and A B towards the ground glass, when used in taking landscapes, or for reproductions of natural size; but when used for enlarging, the combinations are reversed, A B being then towards the object, and E F towards the ground glass. The largest aperture of diaphragm should be used for rapidly taking groups, and seizing on instantaneous effects; but for landscapes and reproductions, when the time of

exposure is comparatively of little importance, small apertures may be used. The diaphragm at L L' contains apertures varying in diameter in proportion to the light required.

When required, C D can be taken out, by first screwing off A B; if the lenses A B and E F only are used, the focal length is reduced one-half, and the rapidity is proportionately increased; but then, though the system may be achromatic, the objective cannot be used for portraits, except in special cases, owing to the great curvature of the field.

When used for taking portraits out of doors, for groups or reproductions, the three combined lenses, as shown in Fig. 65, must be employed.

By the use of an aperture the thirtieth of the focal length (seven inches), brilliant and sharplydefined images are obtained, equal in extent to the focal length. If it be necessary to use a larger aperture, it is only the extent of surface sharply defined that diminishes, but not the sharpness of the image itself. This advantage is so considerable in practice, that the triplet has come into extensive use.

The Flare in Photographic Lenses.—The cause of the "flare" or "central spot" in photographic landscapes and views of buildings had puzzled photographers and opticians for a considerable length of time. By the scientific knowledge of Sir John Herschel, combined with the practical knowledge of Mr. Dallmeyer as an optician, the cause not only has been satisfactorily ascertained, but effectually removed. All double-combination objectives or lenses embracing large angles of view, and used with a small central diaphragm, had the defect, more or less, of producing a bright central spot or flare in the picture. This flare was caused by the surfaces of the back-combination lens reflecting the aperture of the diaphragm.

The following extract from a letter of Sir J. F. W. Herschel to Mr. Dallmeyer explains this phenomenon: "The curious white patch in the middle of your very extraordinary photograph at first puzzled me, thinking it was somehow a phenomenon of diffraction. But on considering your experiment when an image of a candle was produced,* it has just occurred to me that some such image of the diaphragm-aperture might arise

*Mr. Dallmeyer found that, on removing the diaphragm of a lens with which he performed some experiments, and placing a lamp-flame in the exact position thereof, the image of the flame occupied the place on the ground-glass previously occupied by the central spot.

from reflection between the glasses of the lens used. Thus taking the simple case of a single lens, A B E f a, Fig. 65a, placed so as to have c, the centre of the aperture c, in the centre of curvature of the surface A B E, then a ray from c will penetrate that surface perpendicularly as c a b, and meeting the concave surface A FE, whose centre is D, it will be partially reflected at b in the direction b d e, making the angle Dbe Dbc; arrived at d it will be partially again reflected in the direction d c, making the angle x d c = x d b.

Again impinging on the surface a C F in c, it will be

refracted in the direction cf, making the angle y cf greater than D c d, because the reflection is out of glass into air. Thus will rays nearly central converge to a focus f on the convex side of the lens, and form an image of the aperture with more or less aberration; in fact the aberration of such a pencil-cone will be very great, and the rays divergent on the whole of so very large a lens from so near a point will scatter over a large area. I have not calculated the foci after the several reflections, nor the law of the aberrations; but on drawing a figure on a tolerably large scale, and