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

148 THE PHOTOGRAPHIC NEWS. [March 27, 1868. diminishing :— Distance of object. 10,000 metres 1,000 „ 100 „ 50 „ 10 „ 5 „ 3 „ 2 „ 1 50 centimetres 40 30 „ 20 Elongation of focal length of the lens. 0’001 millimetres 0’01 0’1 0 2 1’01 2’04 „ 26 3-5 »> 5-3 , UI 25 33’3 ,, 50 100 very distinctly. If you now direct the glass to nearer objects you will also see them with perfect clearness. Similarly, by holding the opera-glass directed on a distant object, you will see that you can move the eye-piece a short distance without lessening sensibly the sharpness of the image by doing so. It is to be observed that the depth of the focus varies with the aperture of the lens. Figures 44 and 45 make this very plain. In figure 44 we make use of a lens 5 with its entire aperture. The e rays, r r, emanating from a dis- • tant point, form, after having traversed the lens D, the image of the point at a on a screen or ground glass A. But if the ground glass be either drawn • back or advanced, to C or to B, 5 for example, the image of the e point immediately spreads out • in a circle, because angle a is very large. When the same lens, D (fig. 45), is reduced to its central part by a stop, the image of the point is still formed at a, but the ground glass can be placed at C or at B, without the image of the point becoming now appreciably altered. This is because, in fig. 44, the rays, r r, emerging from the lens, are much more convergent than in fig. 45. It results, therefore, from the preceding, that a convergent lens is capable of giving a sharp image of planes distant from each other, contrary, apparently, to the law of conjugate foci. But experiment shows that it is only on the condition that these planes are sufficiently distant from the lens that their image may be formed near the principal focus. Thus, the nearer objects approach the lens, the less becomes this depth of focus, as is shown by the following short table, which gives the focal lengths of a lens of 10 cent, focus, for objects of which the distance is constantly This table is very instructive. It enables us to see clearly that for objects 50 metres distant, for example, the focal length of the lens is increased only two-tenths of a millimetre, a length quite inappreciable; for 10,000 metres it is increased still less, only a thousandth of a millimetre. Therefore, all objects situated more than 50 metres from the lens will be in focus on the ground glass, however great may be their distance. When, on the contrary, the object is situated very near the lens—■ for example, 30 centimetres from it—the ground glass has to be drawn out 5 centimetres ; when at 50 centimetres, the glass has to be drawn out 24 centimetres; when the object is distant one metre, the glass has to be drawn out 11 millimetres—quantities relatively great. For this reason, objects situated at a short distance from the lens give sharp images only on the condition that they are very near each other ; hence the difficulty of obtaining the image equal ly sharp if this condition is not fulfilled. Thus, then, the depth of focus of the lens varies with its aper ture and the distance of the objects which form the image at its focus. It varies, also, according to the form of the lens or the optical combination of lenses composing an objective. Conver gent meniscus lenses have the greatest depth of focus when their concave face is towards the objects. Among objectives composed of several lenses, the orthoscopic has the greatest depth of focus, and the ordinary double combination the least. This is because the former has generally a small aperture in relation to its focal length, and because, moreover, the divergent lens placed along with the anterior convergent lens renders the emergent pencils less con vergent. The second, on the contrary, has generally a very large aperture, and besides, the rays emergent from the first lens (the one which is towards the object) are rendered still more conver gent by the second lens. After giving a full description of the various forms of photographic lenses in use, the mode of using photographic lenses receives attention, after which the author proceeds to the subject of enlargement, a subject he has pre-eminently made his own, and demonstrated by the production of some of the finest enlargements which have ever been brought before the public. In this part of the book the processes and modes of working receive attention as well as the apparatus to be employed, and much valuable information is conveyed to which we shall probably return on a future occasion. In the meantime, we remark that the work is one which every photographer ought to possess and study, and that the photographic world is deeply indebted to Dr. Van Monckhoven for the production of such work, and to Mr. Hardwicke the publisher for presenting it to English readers in an admirably neat and perfect form. VENTILATION OF DARK ROOMS. Mb.. Chas. WAGER Hull, in the Philadelphia Photographer, commenting on Mr. Cherrill's plan for ventilating dark rooms given in our last volume, says :— “ On reading and examining it, I was convinced that the plan alluded to of curing the evil would add another to the long list of the photographer’s troubles, which, though it might save his health in one way, would injure it in another by spoiling much of his work, make him mad, ruffle his temper, cause him to say ‘ naughty words,’ and generally be productive of bad consequences. If we are to have ventila tion in our dark rooms, it is well for us to have not only good wholesome air, but, as well, air not charged with dust, which would be the case if introduced, as is proposed, in the article referred to, and which most certainly would be the case if brought in from out of doors. Imagine a current of air passing into our operating-room in midsummer, when everything is dry, and all the lighter particles go floating about through space, in entire disregard of eyes, nose, ears, or as, in this instance, dark rooms. “ By the introduction of this air into the dark room, I claim that though the physical health of the operator might be benefitted, bis moral health would be injured, his work would be spoiled; therefore, of the two evils, choose the least, of which choice there can be no doubt. ‘ How shall we better this plan ?’ the reader may be asking. Very easily, and thus : Do away with the door of your dark room, and, in its place, build up a crooked sort of an entrance, as seen in the cut following. “ By this simple plan all light is cut off from the entrance, and the room is as well ventilated as need be, without the troubles which are certain to come from the plan first alluded to. “ You enter at A, and, turning to the left, pass through B, and enter at C into the dark room D. By this simple arrangement you do away with the door, the constant open ing and closing of which raises, during a busy day’s work, enough dust to produce pinholes ad libitum. “ To my knowledge, several rooms built after this plan are an unqualified success. If any reader imagines that the light can possibly get into his room (which 1 do not), then let him make his passage B wide enough to put a partition