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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1277, February 23, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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126 THE PHOTOGRAPHIC NEWS. [FEBRUARY 23, 1883. have lain in the potash solution or turpentine a sufficient time to render the body semi-transparent. Sections of woods, if cut thin and strained a suitable colour, make good pictures. Sections of lung, if thin, give good results, but many anatomical preparations of soft tissues are too thick to allow of perfect focussing with the higher powers. A beautiful section of the retina of the human eye which I possess, although most interesting when viewed in the ordinary microscope, is quite unfit for the camera. Diatoms which present a flat surface, like arachnoidiscus, are most suitable, but require careful levelling on the stage. Exposure.—No fixed rule can be given for exposure, which varies with the light, lenses, and length of focus employed, but much trouble will be avoided by the operator if a standard light be used, and a careful record kept of the results of exposures by means of a register, such as that which lies on the table. With Swan’s “ ten times ” dry plates a small microscopic paraffine lamp without condenser, fixed seven inches from the object, gave a good picture in one minute, with a two-inch lens full aperture, and the bellows extended to thirty inches. As a rule, amateurs over-expose their plates, and produce a weak, thin negative. The movable disc before referred to affords an easy method of testing the time of exposure, four tests being obtained on one plate. Another method is to partially raise the shutter at intervals, noting the time for each exposure ; then to cut the plate down the middle with a diamond, and develop one half two or three minutes longer than the other. Six varia tions on the exposures can thus be seen, and the utmost novice will be able to discern “ which way the cat jumps.” Photographic Process.—If sunlight were always at command I should prefer the wet collodion process, on account of the readi ness and rapidity with which trial plates can be developed and examined. Every object differs so much in density or size that the time of exposure is very varying, and carefully-repeated ex periments are necessary. I have heard it stated that Dr. Maddox thought he did well to secure one good negative a day. Development.—For readiness of application at rare intervals of leisure I have preferred the ferrous oxalalate developer for dry plates, and all my gelatine negatives have been executed so far by this process ; but it is doubtful whether this method allows of so much latitude in nursing up the contrasts in a negative of a very transparent object, as is afforded by the pyro developer. I have also intensified, where requisite, with the saturated solution of bichloride of mercury, and, after well washing, steeping in the solution of ammonia. The solution of mercury may be kept in stock and used over and over again. Magnification.—The greater the disparity between the distance of the front lens from the object and the distance of the lens from the sensitive plate the less chance is there of securing penetration or deep focus. Therefore, if large prints are required, better results will follow from employing a low power, and taking a small picture, afterwards enlarging from the negative. Quarter plates will suffice in most instances for this class of work, and those members who have seen Professor Piazzi Smyth’s negatives of the Pyramids, only one inch square, and enlarged three diame ters by Mr. Pollitt, will be aware how well they bear further magnification on the lantern screen. The highest resolution I have heard of or seen by photo-micro graphy is that by Dr. Woodward with Zeiss’s oil immersion lens one-twelfth of an inch, on the diatom Amphipleura pellucida, where the striae, which in nature count about 100,000 to the inch, are plainly delineated on the print; and Mr. Crisp, the secretary of the B.M.S., tells me the lines have never been so clearly shown by ordinary vision. The nineteenth band of Nobert’s test-plate of finely-ruled lines on glass, containing about 100,000 to the inch, was also resolved by this unapproached operator with Tolle’s one eighteenth of an inch immersion lens. From certain late researches of Professor Abbe the theory is established that when we are near the limits of “ resolution ” the superiority of photographic vision, so to apeak, over that of ordinary microscopic vision, is as 5 to 4, all other things being equal; but it must be remembered that ordinary microscopic powers are constructed for vision, and not for chemical por traiture. X 12 „ 21 „ 37 „ 80 4 in. 2 >> 1 » 3 » The various magnifications obtained by tbe several powers are set down by Mr. Davis as follows. At thirty-six inches :— = X 173 „ „ 360 „ 530 -in. 3 » I >> screen some of the In conclusion: before we show upon the combined work of Mr. Pollitt and myself, allow me to admit that in the presence of such an important Society 1 feel I have much more to learn than to teach, and I invite your candid criticism of any statement I have made with a view to furthering the develop ment of a most interesting branch of your art, which for some years has been a source of much interest to myself, and is evi dently about to become of great educational value. Uorresponbence. THE PHOTOGRAPHIC CLUB AND PUBLICATION “ Oh, dear ! I heard such a shout, Asking for reformation which they can’t do without.” Sir,—As one of the fathers of the Photographic Club, permit me to thank my children for the compliment they have paid me, and to congratulate them on their adopting at least a part of the reform I so strongly advocated—viz., publishing a programme of the proceedings, a free meeting monthly, &o. Some of my friends have likened me to Arabi Pacha, who, after having been kicked out, his enemies have seen fit to introduce the required reforms. I am glad to see that at last something is likely to be done to “warrant the chairman in certifying that the matters of discussion are fit for publication (being a rule of the Club).” I can scarcely think that for two years a dozen persons have met weekly without something being said or done worthy of puhlication. Apologising for intruding on your space, but in the inte rests of the profession, I think it right that this matter should be ventilated.—I remain, yours respectfully, A. L. Henderson. THE TRANSIT OF VENUS. Dear Sir,—Please correct a little mistake in your report respecting photos of “ Transit of Venus ” in your last number. I did not say that the photo. No. 3 presented the appearance of a deeply-cogged wheel with curved teeth, but that it was possible that, owing to the planet so appearing in the telescope, it might have affected the size of the image on the plate.—I am, dear sir, yours truly, J. Foster. o Proceedings of Societies. Newcastle-on-Tyne and Northern Counties’ Photographic Association. The ordinary monthly meeting of the above Association was held in the College of Physical Science, Newcastle, on Tuesday, the 13th inst., at 7.30 p.m., Mr. A. L. Steavenson in the in the chair. The minutes of the annual meeting were read and confirmed. Aletter was read from the President (ColonelSheppee) expressing regret at not being able to be present; also the following letter— “ University of Durham, College of Physical Science, Newcastle-on-Tyne, Feb. 6th, 1883. “ Dear Sir,—1 have much pleasure in stating, that the por trait of the late Professor A. Friere Marreco, which was so kindly presented to the College of Physical Science by the Newcastle- on-Tyne and Northern Counties’ Photographic Association, was formally placed before the Council of the College at their meeting yesterday, and I was desired to inform you that the same was accepted with much gratification, and to beg that you will be good enough to convey the thanks of our Council to your Association for their very kind consideration.—Believe me. dear sir, yours very faithfully, Theo. Wood Browning.” Messrs. J. Garland, E. Schumann, and R. Snowdon were nominated for membership. Mr. Edgar Goold, of Elswick, gave a practical demonstration of Marion’s Ferro-Prussiate Process ; and also of Pellet's Patent Direct Positive Process (seejpage119). The operations, skilfully performed, were witnessed with much interest by the members present, and at their conclusion a hearty vote of thanks was accorded Mr. Goold on the motion of Mr. Payne, seconded by Mr. Pike.
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