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44 THE PHOTOGRAPHIC NEWS. [January 16, 1891. spherical surfaces through polishing, with the result of thus obtaining pretty satisfactory images. By means of our method of annealing, we have succeeded in producing discs for object-lenses having a diameter up to 35cm. nearly perfectly free from tension ; the entire surface of the disc being made to become efficient under the polaroscope. All that is necessary during the test is to exclude any differences of temperature of the discs, as these are apt to give rise to temporary tensions. Nearly all discs annealed according to the older system show the distinct black crosses characterizing the pre sence of strain and pressure, even in those cases where the diameter of the discs does not exceed 2 cm. Though it must be admitted that many opticians, before grinding large-sized lenses for telescopes, will ascertain the properties of the glass with respect to annealing, yet we know from experience that there is consider able inclination to underrate the serious effects of tension, and that many go so far as to consider exami nation before or after making a lens hardly worth the trouble. We append to these lines a sketch of an apparatus, the principle of which is due to Prof. Mach, of Prag, which may be readily put up, and by means of which plane plates (fig. 2) or positive lenses (fig 1) may be tested. The apparatus must be adjusted in such a manner that with parallel nicol prisms the eye at E sees the lens or disc to be tested fully illuminated : if now the nicol prisms be crossed, total extinction will take place with glasses having no internal tensions, whereas with imper fectly annealed glasses the well-known figures indicative of tension will present themselves. In order to examine the figures due to strain or pres sure in all positions of the prism with respect of the disc of glass, it will be found advantageous to turn both prisms synchronically rather than to turn the discs them selves, as these, owing to the touch of the warm hand, may become locally heated. In order to better distinguish the present new method of annealing from that hitherto in use—raw annealing— we have introduced the term “fine annealing” when referring to the former. Our experiments and improvements made with regard to the process of glass-annealing, have induced us to adopt for our fabrication the well-known plan, worked in Paris for many years, of moulding the glass by means of pres sing it while in a semi-liquid state between metal cups having as nearly as possible the same curvatures as the lens. Lenses produced in this manner are utterly useless for application in better class instruments if the ordinary quick process of annealing be employed, as the internal strain in the glass will generally be very great, and some times may become so excessive as to cause the rough lens to fly into small pieces as soon as an attempt is made to grind it. However, with our new method of annealing, which permits of annealing at such low temperatures as to put deformations out of the question, ready means are furnished to produce lenses of this kind entirely free from internal strains by subjecting them, after having allowed them to cool down, to a second process of an nealing in the above mentioned apparatus. Wishing to remove a prejudice held by many opticians, we will not omit to distinctly state that the pressure exerted on the glass while in a semi-liquid state is by no means the cause of internal strain or pressure; on the contrary, the only source of these must be looked for in the accelerated process of chilling which has to be made use of in order to prevent deformation. The favourable practical results obtained by many opticians with glass prepared in this manner encourage us to recommend its application for all such cases where large numbers of lenses of the same kind have to be made with various curvatures and diameters within the limits between 12 to 120 mm. The increased costs of glass prepared in this manner are amply compensated for by the saving of material and work. Lenses having the exact form of the desired lenses suffice as patterns. On account of the prolonged period of annealing, we require, as a rule, a term for delivery from six to eight weeks. We are prepared to submit sample lenses to our customers. Fig. 2. A Illumiqatinz sburce (bright-burning petroleum flime). B Polarising nicol prism. C Lens or disc to be tested. D Analysing prism. J? The observer’s eye. •S' Concave mirror. In fig. 1 A and Bare conjugative points with respect to the lens C; in fig. 2 both points are situated in the plane of the centre of curvature of the mirror .S'. ErKoxoGEN Development.—According to a recent communi cation of M. I. Jullien, photographer, of Geneva, the best pro cess for development is the following : Place for a minute the exposed plate in the carbonate of soda bath or the carbonate of potash bath, withdraw it and plunge it in a bath contain ing the eikonogen and the sulphite. The carbonate of soda enters into the pores of the gelatine, and the quantity con tained in them on coming from the first bath is more than sufficient to facilitate the reduction. This method gives good results, it seems—better than any hitherto obtained.—Revue Suisse de Phot. American Photographers and Tariffs.—Photographers have all learned of the new “Tariff Bill,” and the majority have become informed by their enterprising stock-dealers of the increase of price of some of the principal staples used in the production of photographs. At first sight—“in these times when the prices for photographs are so ruinously low ” —all this appears rather dismaying to the average photo grapher. It need not dismay him, however, unless he wills that it should do so. There is a bright side to it all, and he can gradually and surely work over to it if he wants to. For a long time there has been a continuous howl about “rough competition” and “cut rates.” Now, the first step over to the bright side, after you begin to believe there is one, is to ignore the competition and raise the rates. Two good advances are thus made. We do not advocate a large advance in prices. If fifty cents per dozen were added all over the country, it would secure more than double enough to pay the “extra tariff ” on albumen paper, nitrate of silver, eikonogen, and so on. Then you could stop “howling,” for there would be no thing left to howl about. It is true your patrons would howl a little, but—let ’em. They will soon stop, provided you keep a neat establishment, work alongside of the growth of your art, pay for your Magazine promptly for 1891, and make the best of work. — Wilson's Photographic Magazine.
