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January 16, 1891.3 THE PHOTOGRAPHIC NEWS. 43 A NEW HAND CAMERA. MOULDING PHOTOGRAPHIC LENSES. Messrs. Mawson & Swan have exhibited to us a new A communication from the Glastechnisches Laboratorium, A plane mirror, not shown Fig. 1. hand-camera of Dutch origin, manufactured by Messrs. Loman & Co., of Amsterdam, and which possesses two leading features, one of which is that the theoretically excellent roller blind shutter next the plate is employed, and the other is that the image to be photographed is seen of full size and in sharp focus the instant before making the exposure. Variations of focus can be made by the aid of the ground glass screen. The camera is made in different forms, the simplest of which is pictured in fig. 1. in the cut, nor mally rests at an angle of 45°, and reflects on to the ground glass at the top in the old camera-ob scura fashion. It has also one of the old- fashioned shades which, when not in use, lies flatly on the top of thegroundglass. When the image is in true focus upon the screen, it is in equally true focus on the plate when the mirror is removed. The roller-shutter behind the mirror can be set to any speed. The lens, in the camera of this pattern, is a rapid rectilinear, fitted with rack-and-pinion. Fig. 2 represents a camera of the same class, but more expensively finished. f/5*6, and we are told that it gives good definition even at this aperture, and, as it has an iris diaphragm, smaller aper tures can be used if re quired. The instruments The lens is inside, and works at are worth ex Fig. 2. amination by those who are selecting a hand-camera, and Schott and Gen., in Jena, published in the American Journal of Photography, deals with the influence of the process of cooling upon the optical properties of glass, and the production of pressed lenses in a thoroughly annealed state, and says that the very imperfect state of annealing generally met with in the glass-discs for larger telescopes formed for years a constant source of com plaint from opticians who, in the manufacture of large sized lenses, are working strictly spherical surfaces. For this and other reasons, it has been our aim ever since the erection of our works to improve the process of annealing. The method hitherto in use, viz., that of allowing the temperature of the red- hot glass to fall in a kiln completely enclosed by brick-work, which gradually transmits the heat stored up in it to the surrounding atmosphere, has been discon tinued by us in all cases where high optical properties are aimed at, and in its place we adopted the plan of storing the glass in a vessel the temperature of which may be accurately measured, and subjected to a very slow and strictly uniform decrease, the duration of which may be adjusted to suit special requirements. The experimental researches which formed the necessary predecessors of our new process of annealing, offered ample opportunity for minutely studying the influence of internal strains and pressures upon the optical properties of the glass. We intend to treat in extenso on this sub ject at a later date ; here it may suffice to mention the most important of those results which will interest practical opticians: 1. Any kind of glass becomes strained—i.e., the mole cules of the glass are subjected to tension—unless the process of solidification be extended over a very long period. 2. The refractive index of one and the same piece of the glass varies according to the duration of the process of annealing; this diversity may extend to several units of the third decimal place. 3. If a lens or circular disc, on being carefully examined by means of polarised light, be found to yield a regular black cross, which remains perfectly free from any distortion during a complete rotation of the disc about the optic axis, it may be inferred that the tension is strictly regular throughout the entire piece of glass under examination. The presence of a moderate tension of this kind has no other effect as if there were a slight gradual diminution of the refractive index in the direc they present some features which are theoretically good, and should be of practical value. REPRODUOIIOX of Positives in Original Size.—When the bellows of the camera which we have at our disposal does not give us the required size, one may proceed as follows : Place in the pressure-frame an ordinary gelatino-bromide dry plate, then immediately the original on top, so that the image be in direct contact with the sensitive film. It goes without saying that if the print has been stuck on a mount it must be removed from it. Place over the whole another glass plate, then expose in such a manner that the light only reaches the sensitive plate after having passed through the glass and the print. The ex posure lasts in diffused light from one-half second to five seconds. The negative is developed as usual, and we obtain a perfectly sharp reproduction without a shadow of granulation, as might be feared, on account of the interposition of the paper. Per Amateur Photograph. tion of of the axis. Owing to the symmetrical arrange ment of the tensions round the axis, they do not exercise any detrimental influence on the image. 4. If, however, a lens or circular disc, while being turned round its axis under examination in polarised light, show in any one or several positions a displaced black cross or any other irregular figure, the tensions must be considered to be irregular. The influence of such tensions dissymmetrically grouped round the axis is identical to that of a difference of the refractive power in different parts of the lens. Glasses of this kind should never be employed for the manufacture of large-sized objectives. With telescopic lenses made of glass where this defect existed in a moderate degree, many opticians attempt to compensate this differentiation of refraction | by introducing at random deviations from strictly