The photographic news

• Bandzählung: 35.1891
• Erscheinungsdatum: 1891
• 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
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• PURL: http://digital.slub-dresden.de/id1780948042-18910000
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• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

January 2, 1891.] THE PHOTOGRAPHIC NEWS. 13 ASTRONOMICAL TELESCOPES* BY A. A. COMMON, F.R.S., TREASURER TO THE ROYAL ASTRONOMICAL SOCIETY. Commencing, then, with the first application of the telescope to the investigation of the heavenly bodies by Galileo in 1609, we find that the largest telescope he could make gave only a magnifying power of about 30. The first improvement made in the telescope, as left by Galileo, was due to a suggestion—by some attributed to Kepler, but certainly used by Gascoigne—to replace the concave eye-lens that Galileo used by a convex one. Simple as this change looks, it makes an important, indeed vital improvement. The telescope could now be used, by placing a system of lines or a scale in the common focus of the two lenses, to measure the size of the image produced by the large lens ; the axis or line of collima tion could be found, and so the telescope could be used on graduated instruments to measure the angular distance of various objects; in fact, we have now, in every essential principle, the true astronomical telescope. It is useless as an ordinary telescope, as it inverts the objects looked at, while the Galilean retains them in their natural position. The addition, however, of another lens, or pair of lenses, re-inverts the image, and we then have the ordinary telescope. It was soon found that the single lens surrounds all bright objects with a fringe of colour, always of a width of about one-fiftieth of the diameter of the object-glass, as we must now call the large lens; and, as this width of fringe was the same whatever the focal length of the object-glass, the advantage of increasing this focal length, and so getting a larger image without increas ing the size of the coloured fringe, became apparent, and the telescope therefore'was made longer and longer, till a length of over one hundred feet was reached ; in fact, they were made so long that they could not be used. A picture of one of these is shown, from which it can be easily imagined the difficulties of using it must have been very great, yet some most important measurements have been made with these long telescopes. Beyond the sug gestions of Gregory and Cassegrain for improvements in the reflecting telescope, little was done with this instru ment. During the eighteenth century immense advances were made in both kinds of telescopes. With the invention of the achromatic telescope by Hall and Dollond, the long- focus telescopes disappeared. Newton had turned to the reflecting telescopes, believ ing from his investigations that the dispersion and refrac tion were constant for all substances; this was found not to be so, and hence a means was possible to render the coloured fringe that surrounds bright objects when a single lens is used less prominent, by using two kinds of glass for the lens, one giving more refraction with some what similar dispersion, so that while the dispersion of one lens is almost corrected or neutralised by the other, there is still a refraction that enables the combination to be used as a lens giving an image almost free from colour. In 1733, Hall had made telescopes having double object glasses on this plan, but never published the fact. Dol lond, who had worked independently at the subject, came to the conclusion that the thing could be done, and suc ceeded in doing it; the invention of the achromatic telescope is with justice, therefore, connected with his name. * Continued from page 1013, vol. xxxiv. Although this invention was a most important one, full advantage could not be taken of it, owing to the difficulty of getting discs of glass large enough to make into the compound object-glass, discs of about four inches being the largest diameter it was possible to obtain. With the reflecting telescope, unhampered as it always has been by any except mechanical difficulties, advance was pos sible, and astronomers turned to it as the only means of getting larger instruments. Many most excellent instru ments were made on the Newtonian plan. The plan proposed by Gregory was largely used, as in this instru ment objects are seen in their natural position, so that the telescope could be employed for ordinary purposes. Many were also made on the plan proposed by Casse grain. The diagrams on the wall enable you to at once see the essential points of these different forms of re flectors. About 1776 Herschel commenced his astronomical work ; beginning with reflecting telescopes of six or seven inches, he ultimately succeeded in making one of four feet aperture. With these instruments, as everyone knows, most brilliant discoveries were effected, and the first re al survey of the heavens made. Herschel's larger telescopes were mounted by swinging them in a surrounding framed scaffolding that could itself be rotated. The smaller ones were mostly mounted on the plan of the one now before us, winch the Council of the Royal Astronomical Society have kindly allowed me to bring here. The plan nearly always used by Sir William Herschel was the Newtonian, though for the larger instruments he used the plan proposed years before by Le Maire, but better known as the Herschelian, when the observer looks directly at the large mirror, which is slightly tilted, so that his body does not hinder the light reaching the telescope. In all cases the substance used for the mirrors was what is called speculum metal. (To be continued.) patent Intelligence. Applications for Letters Patent. 20,438. R. W. Thomas, 70, Chancery Lane, London, “Flash- Light Apparatus for Photography.”—December 15th. 20,452. A. Edington and A. Daniell, 3, Great King Street, Edinburgh, “Cameras.” 20,501. S. H. Fry, Minerva Road, Kingston-on-Thames, “ Holding and Exposing Sensitive Photographic Surfaces.” —December 16 th. 20,563. Sarah Tucker and W. J. Spurrier, Cotton Lane, Moseley, near Birmingham, ‘ ‘ Magazine Hand Camera. December 18 th. 20,774. J. Pitt and W. Hudson, 118, Gooch Street, Bir mingham, “Photographic Dark Slides.”—December20th. Specifications Published. 18,689. November 21st, 1889.— “Washing Photographic Prints.” Birt Acres, 131, Richmond Road, Hackney, Lon don. My invention relates to the washing of photographic silver and other prints ; the object of my invention is that the prints may be kept in an upright position in a tank or other suitable vessel of water, in such a manner that they could not stick together, double up, become crumpled, or in any other way damaged. To secure this object I construct two light frames of wire or any other suitable material (if of metal, protected by a coating of water-proof enamel, paint, or varnish), a little larger than the size of the print intended to be washed, the top of one of the frames having a shoulder projecting at each end to