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The photographic news
- Bandzählung
- 29.1885
- Erscheinungsdatum
- 1885
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- Englisch
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1380, February 13, 1885
- Digitalisat
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Zeitschrift
The photographic news
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Band
Band 29.1885
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- Register Index III
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Band
Band 29.1885
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FEBEUAIY 13, 1885 J THE PHOTOGRAPHIC NEWS. 99 lenses, we will enquire later on. But first let us see how it is made, and readers may perhaps like to unscrew their back lenses and follow my description. So far as I am aware, the construction has always in cluded a front combination in which the anterior lens is crown or plate glass ; the form of the whole cemented com bination being plano-convex in the earlier forms, or a shallow meniscus in later ones. This form, very similar to the telescopic object glass made on Clairaut’s curves, is the easiest to adjust, and the cheapest to construct; slight errors in workmanship interfering less with the correction of direct aberration than in the doublet form of a deep meniscus with anterior flint lens and deep interior curves, as this is again more favourable to oblique pencils. In the back combination, the flint concave presents a convex side to the stop, and a deep concavity towards the double con vex crown or plate-glass behind it, which it achromatises. Of course the whole set of curves and distances are so arranged as to balance the direct spherical aberrations, and that, as is well known, to extreme perfection, as is proved by the large aperture permitted. To understand this, we will first remark that the amount of aberration to be generally expected may be conveniently summarised in the approximate rule that" the aberration at any surface varies nearly as the cube of the refracting angle.” Thus, if all the refractive power of any lens is exerted at one side only—as, for instance, on the convex side of a plano-convex lens, the plane side of which, turned to parallel rays, transmits them without deviation—and we reckon that refractive power as equal to 4 ; then, if equally divided, as by divergent incidence, or convergent emergence at that plane surface, they will equal 2 at each surface. Now the cube of 4 is 64, while twice the cube of 2 is 16, only a fourth part, so that equally dividing the refraction in place of throwing all on one surface, reduces the aberra tion to a fourth of its value. Though this is not quite exact, the ratio varying both with the glass and with the curve, it is yet a good approximation, expressing very well the general mode of dealing with aberration by altering the proportions of the curves, and the general advantages of dividing the refractions. For if the original error is small, errors of workmanship and adjustment are of less consequence. As to errors of obliquity, which no adjust ment can entirely correct, division of refraction, and avoidance of violent curvesis still more important, though, unfortunately, they divide differently; a divergent bundle of convergent pencils, or a convergent bundle of diverging ones, being an inevitable arrangement, and, as we shall see, it is in the judicious management of oblique aberrations that flatness of field is obtained. We may now see that the excellent central definition of the Petzval lens is attained easily, because none of the curves arc violent in relation to the form of the trans mitted pencil. The general advantages of this form in correcting direct pencils, and those of doublets in the trans mission of oblique ones, may be now seen by simple in spection. A general comprehension does not really need any very recondite study or elaborate calculation ; abun dantly necessary, and highly complicated as are these investigations and calculations to combine all the required conditions to the best attainable compromise, and de duce working formulas. I venture to hope that an in telligent appreciation is more generally attainable than is sometimes supposed. For if we place a doublet to copy any picture, we may see that a bundle of rays starting from any point near the margin of the object, to be converged by the lens to a corresponding point in the image, passes through each surface favourably as regards the obliquity of its incidence, the ideal condition being that tiie axis or central ray of each pencil shall have a direct perpendicular incidence and emergence. If a stop be used in the centre, we can see at once by inspection that each bundle of rays passes through a small ci. cle on each lens, harmoniously placed at each surface with regard to its general direction. No elaborate demonstration is required to perceive this, or to perceive how inharmoniously the corresponding small circles on the back lens of a F'etzval combination appear, as the pencils spreading out from the stop fall obliquely on the surfaces of the flint lens. Of course this favourable incidence is not everything in a doublet, or else a globe lens would be as good as a wide angle rectilinear. But to return to our direct pencil: in a Petzval back lens we see that a central pencil, rendered convergent by the front lens, enters first a convex surface whose direct action on it is almost nil. and emerges from a deeply concave one still not very unfavourable to the direct action. Then, having gener- ally been rendered moderately divergent, itpasses through a lens convex on both sides, which form is seen by inspection to be favourable for dividing the refractions, and thus diminishing the aberration of a pencil which, diverging at incidence, is converged at emergence. No one surface presents so violent an angle as to throw the refraction wholly on one side, except the deeply concave one ; whereas in other constructions, we may see the refraction strongly divergent atone side and convergent on the other of the same lens. Here all the direct aberrations are comparatively moderate at each surface ; and this, no doubt, is the secret of its combined excellence and cheap ness in proportion to aperture. (To be conti’iucd.') THE NEW RAPID PRINTING PAPER. Note on the Exposure. by WALTER d. WELFORD. It may, perhaps, appear rash on my part to write any thing about the new process which is to revolutionise photography, so tar as the multiplication of copies is con cerned, the thing being so new, and, as I believe, so capable of improvement. But I have had rather a unique experience, and the relation of it may aid those who have either not yet tried the process, or who have but slightly dipped into it. An order was in view, 500 4x3 prints to turn out in four days, and I knew it meant using the new process. First assuring myself that paper was really obtainable in this town, I accepted the order and felt happy—for a time. The instructions revealed no great difficulty, and thirty to thirty-five seconds’ exposure to a good gas light, with negatives of ordinary density, ap peared an easy matter. To ensure “ ordinary density,” I developed with ferrous oxalate, and took care the nega tives should be well exposed, and thin. From the instruc tions it seemed to me that under-exposure would likely be the cause of failure, so instead of thirty-five seconds, I gave the first trials sixty seconds. On developing they came up, as I thought, well, and until placed in the toning- bath things were going as merrily as a marriage bell. But the bath had no effect, and the results were of a very pretty olive green—not at all bad to look at, but, as I had to match prints on albumenized paper, of no earthly use. “Double the exposure will do it,” said I, and the next night I gave two minutes to two and-a-half. What made me trust the paper so implicitly I know not; but true it is, I exposed over 150 prints. One maker's instruc tions stated that the prints would take three to five minutes to develop, and as I was certain my first trial did not ex ceed that, I felt sure doubling the exposure would obtain the brown tone. When I set to work, however, I was horrified to find exactly the same results. The toning bath was blamed at once; still, neither fresh baths nor fresh chemicals had any effect, and 1 felt considerably fixed. Application to the makers of the paper brought me no comfort, as they merely stated they could not under stand it, and asked me to send a sample of the paper ; and though a piece was at once sent—of course the time male it imperative —I should find the fault out myself. Every thing received its due share of blame —the poora’um, iron,
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