The photographic news

• Bandzählung: 12.1868
• Erscheinungsdatum: 1868
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
• Rechtehinweis: Public Domain Mark 1.0
• URN: urn:nbn:de:bsz:14-db-id1780948042-186800009
• PURL: http://digital.slub-dresden.de/id1780948042-18680000
• OAI: oai:de:slub-dresden:db:id-1780948042-18680000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

June 12, 1868.] THE PHOTOGRAPHIC NEWS. 281 The PREPARATTON of Drawings, etc., for Reproduction. One of the most useful applications of photo-zinco- graphy is the reproduction of maps and civil and military engineering drawings ; but as the requirements of the pro cess are but little known, these drawings are seldom drawn in a suitable manner for reproduction by photography, con sequently the results are unsatisfactory, and the process is blamed undeservedly. If care is taken to select suitable | subjects, and to secure a good negative, results can be obtained which may compare with engravings in sharpness and delicacy. Success mainly depends upon the negative, which must be perfectly sharp, free from distortion, and possess the greatest amount of contrast between the lines and the ground, the lines being as transparent as the bare glass, the ground almost opaque. Attention to the following points will greatly lessen the labour of the operator, ami conduce to success :— 1. The drawing-paper should be as white, clean, and smooth as possible. If the originals are on rough paper they should be passed through a copper-plate press, and, if necessary, cleaned with rubber or bread. 2 The Indian ink with which the drawing is made should be freshly rubbed down, quite black, free from grit and glaze. 3. The lines should be firmly drawn, and pale ink must on no account be used. The marginal lines must be well filled in. 4. Washes of any colour, except very light blue, are in admissible, but outlines may be put in with dark burnt sienna, crimson lake, dark green, and similar colours, which will reproduce black. 5. When the plans are intended for reduction, care must be taken to draw the lines of the proper thickness rela tively to the scale of reduction ; that is, supposing it is required to reduce a drawing to one-fourth the size, it will be necessary to draw every line of the original four times as large as will be required in the copy. This rule is often neglected, and the result is the loss of all the finer lines. The best results ere obtained when the drawings are pre pared on purpose for reduction, and without any regard to clumsiness of appearance. 6. When practicable, the drawing should be left on the drawing board, so that the paper may remain perfectly flat, or should be mounted in such a manner as to secure flatness. This cannot be done by pinning the drawing to a board ; the alterations of temperature will affect the paper, and cause it to expand unequally, and produce ridges. I think it will be found better to fasten the drawing down with tapes, fastened with drawing pins outside the margins of the drawing, and passing tightly over it, so as not to check any lateral expansion. It is generally better to copy by strong diffused light, as then there is less danger of reproducing the grain of the paper, the removal of which necessitates considerable rein- tensification to the certain detriment of the negative. Old discoloured manuscripts, &c., are better copied in sunlight, taking care that the sun shines directly on the subject. (To be continued.) THE DIAMOND. BY JAMES MARTIN. As the diamond is now one of the most useful accessories in the photographer's catalogue of implements, it would, no doubt, be interesting to your readers to learn somewhat of its history and application to the arts. The diamond has, from the remotest antiquity, been prized as the most valuable —or, more properly, the most costly—substance in nature. The reason of the high esteem in which it was held by the ancients was its rarity and its extreme hardness for the art of cutting and polishing ; this gem not having been then invented, its superior brilliancy and lustre would not have been appreciated. They also considered it an antidote to poison, and that it was able to cure insanity; therefore it was called, by some, anachitis. Its supposed occult qualities and superstitious uses no doubt contributed greatly to the high esteem in which this substance was held as being the most valuable and beautiful of gems. It was endowed with these hidden virtues in the highest degree ; hence it was held to be an infallible specific in many diseases, and, amongst other absurdities a test of conjugal fidelity, a reconciler of domestic strife, and an amulet of highest power against poisons, insanity, witchcraft, incantations, goblins, and evil spirits. The diamond is either colourless, or of a light yellow or smoke-grey, passing into bluish or pearl-grey or clear wine colour, on the one hand deepening into clove-brown, and on the other into yellowish-green. It also occurs of a deep, almost black-brown, Prussian blue, or rose red ; and the colourless varieties are the most esteemed, and, next to these, the blue, red and black, the light-coloured the least. The diamond is found crystallized in the regular octohedron, which is its primitive form, composed of two four-sided yramids opposed base to base, or in the cruciform octo- idron. Sometimes each triangular face of the primitive octohedron is replaced by six secondary triangles, bounded by curvilinear lines, in which case the whole crystal has forty-eight faces, and is of a spheroidal figure. Other spheroidal varieties of this mineral are the duodecahedron, a solid of twenty-four faces, and a compressed spheroid re sembling a very short hexahedral prism terminated by very short, curvilinear pyramids. The surface of the natural crystal, especially of the spheroidal, is somewhat dull and chatoyant; this appearance, which is generally represented as the effect of a thin crust, appears to be caused merely by the salient edges of the lamins of which the crystal consists. When its surfaces are reduced to perfect smoothness by grinding and polishing, the diamond is of extreme bril liancy, far surpassing every other substance in lustre and the lively play of prismatic colours which dart from it in lines of light whenever its position with regard to the eye undergoes the least variation. The fracture of the diamond is straight foliated ; hence it may readily be cleft in the direction of its lamin by a dexterous artist. Some of the spheroidal varieties, however, are composed of curved plates ; these are of intense hardness, and cannot be either split or highly polished ; they are therefore used by the glaziers and engravers on gems, or are ground into a powder and employed in polishing other diamonds. The specific gravity varies from 3518 to 3'550. The diamond, even when rough, acquires by friction the vitreous or positive electricity; it becomes phosphorescent when exposed either to the entire rays of the sun, or to the blue rays alone when separated by the prism and concen trated on the diamond by means of a lens. The diamond when heated to the temperature of melting copper, and exposed to a current of air, is gradually but completely com bustible. It is surrounded by a luminous areola during the process. It is wholly converted into carbonic acid, and, therefore, consists of pure carbon. The art of cutting and polishing the diamond was probably known to the artists of Hindostan, and at a very early date, but the only material used in the East for this purpose being wumdum, and the apparatus being of extreme simplicity, the jewellers of those countries are incapable of bringing out the peculiar beauty of the diamond in a degree at all comparable to what is effected by European artists. Formerly diamonds were set in jewellery precisely in the state in which they arrived from India, and hence the octa hedrons were much more esteemed than the rest, both on account of the regularity of their figure and the superiority of their polish. Diamonds are cut and polished by jewellers into brilliants and rose diamonds ; the former being for the most part made out of the octohedral crystals, and the latter from the sphe roidal varieties. In the formation of either brilliant or rose diamond, so much is cut away that the weight of the polished gem is not more than one half that of the rough crystal out of which it is formed; whence the value of a cut