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The photographic news
- Bandzählung
- 29.1885
- Erscheinungsdatum
- 1885
- Sprache
- Englisch
- Signatur
- F 135
- Vorlage
- Hochschule für Grafik und Buchkunst Leipzig
- Digitalisat
- Hochschule für Grafik und Buchkunst Leipzig
- Digitalisat
- SLUB Dresden
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- Public Domain Mark 1.0
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- urn:nbn:de:bsz:14-db-id1780948042-188500006
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- http://digital.slub-dresden.de/id1780948042-18850000
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- oai:de:slub-dresden:db:id-1780948042-18850000
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- Seite I-II fehlen in der Vorlage. Paginierfehler: Seite 160 als Seite 144 gezählt.
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- Parlamentsperiode
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- Bandzählung
- No. 1391, May 1, 1885
- Digitalisat
- SLUB Dresden
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- Paginierfehler: S. 287-288 als S. 269-270 gezählt
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- Ausgabe
- Parlamentsperiode
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- Wahlperiode
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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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- Titel
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four spectra showed at a glance the respective dispersive energies of the four prisms, and the spectrum formed by the densest prism appeared to the eye about two-and-a- half times as long as that formed by the prism made of very light glass. Next in order came an explanation of the direct vision prism, and although this instrument is very convenient for class and lecture work, it is of comparatively little use in the laboratory of the investigator, more especially when the records are to be made by photography. Apart from the fact that the complex direct-vision prism has a low dispersive power in proportion to the amount of glass through which the light must pass, it is subject to the serious disadvantage of possessing many reflecting surfaces, and from a photographic point of view this means diffused light and fog. “ What term can be more indefinite than glass?” asked the lecturer, as he pointed out the desirability of making photo-spectral experiments with prisms consisting of definite materials, bodies have specific chemical formulae. Rock crystal is a definite compound of silicon and oxy gen, and it will allow many rays to pass which are cut off by glass, especially rays in the ultra-violet. A spectrum made by passing the light of the electric arc through a rock crystal prism was now projected upon the screen, and the ultra-violet rays were made visible by allowing them to impinge upon fluorescent substances, quinine and heavy petroleum being selected for this purpose. It is scarcely necessary to say that in the case of this experi ment, the condenser and the object-glass were also made of rock crystal or quartz. Considering that many of the ultra violet rays, which are cut off by glass, are allowed to pass freely by rock- crystal, one might suppose that there would be an enormous advantage in making photographic objectives of this latter material; but the advantage of quartz is inconsiderable when sunlight is used, although considerable in the case of the electric light. If an optician were making an objective for especial use in a studio where the work is done by the electric light, he might perhaps use quartz with advantage. Tn certain researches on the ultra violet spectrum of the electric arc, quartz prisms and lenses are essential. Iceland spar, which consists of pure carbonate of lime, yields better results in the ultra-violet than glass, but not such an extended spectrum as can be obtained by the use of quartz, and the value of Iceland spar prisms was illus trated by the exhibition of some of the wonderfully perfect stellar spectra obtained by Dr. Huggins. When light is reflected from a surface of pure silver, there is a considerable loss of the ultra-violet rays ; but as the loss occurs mainly in the extra solar rays, there is not much practical disadvantage in using silver reflectors for work executed by daylight; but when electric light is reflected from silver, there is a notable loss as regards actinic power. The development of a plate by white light was then de- monstrated, the white light being made up by combining a particular tint of the spectral orange with a portion of the blue. This experiment is practicable with an iodide or a chloride plate, but would not succeed with a bromide plate, as the blue would set up an action. Another white light may be made by combining red and green, or any other complementary colours ; but of course such forms of white light must not be confounded with white light as we ordinarily experience it. A colour chart affords a very simple and rapid means of analysing a compound light, as it is obvious that only those colours can bo seen which are present in the light. The slit for photo-spectral work may ordinarily be about v3„th of an inch wide, and a magnified image of such a slit was exhibited on the screen; after which the Lecturer demonstrated the means adopted for superimpos ¬ ing spectra, and taking a number of spectra on the same plate for making comparisons. The rack-and-pinion slide in which the plate can be made to move to a known extent after each exposure, was shown ; no less than six teen photographs of spectra being occasionally taken on the same plate. Afterwards attention was called to the form of slide used for exposing a plate in any required gas or liquid, the plate being included in a glass vessel fitted with an air-tight cover, and provided with tubes for filling or emptying. Diffraction gratings were next discussed, and several very fine ones were shown, the most notable being a con- eave grating made by Professor Rowland, and measuring 6 by 34 inches, and the great dispersive power and remark able dividing capabilities of this instrument were illustrated to the audience by projecting the mixed spectra of zinc and copper on to a transparent screen placed between the Lecturer and the audience. The full text of the lectures will be printed before long. THUMB-NAIL NOTES. (The Grosvenor Gallery.) The exhibition this year at the Grosvenor is rich in por traits, and, what is worthy of note, most of the portraits are good. On the look-out for points of special interest, I took particular heed of the treatment, in several notable instances, of the hands, and found much to profit the photographer. Mr. Millais’ grand portrait of Mr. Glad stone at once fixes the attention in this respect. The hands are here kept well in shadow, and yet are sufficiently marked to catch the eye, and perfect, by the firmness of the muscles, the picture of nervous energy impressed on the face. Had the artist chosen to hang them limply, or crumple them loosely, the painting would have lost half its force. Brought together as they are, one hand clasp ing the other, not carelessly, but as though enforcing the determination of the face, a complete harmony is attained. Mr. Frank Holl’s portrait of Mr. William T. Palmer, meritorious in many respects, is weak in regard to the hands. The right is resting on the arm of the chair, the left is supported by the knees. Of equal force and size, and too widely separated, they are thereby brought into undue prominence, and the face suffers in consequence. Mr. Holl’s Lord Overstone has not this fault, but as the picture is obviously unfinished, it would not be fair to criticise it. All Mr. Herkomer's portraits will repay the study bestowed upon them. The pose of each is easy without being vulgar, the painting is vigorous—almost too vigor ous in the case of Mr. C. Villiers Stanford, the musical composer—and one feels that the artist has caught the true expression of the sitter. Mr. Stanford is standing in what a photographer would call a risky attitude. It is almost a full-face view, and both hands are in the trousers pockets. No attitude could be more life-like, no attitude could be more commonplace. Yet there is nothing commonplace about Mr. Herkomer’s picture. Photographs with one hand in the pocket are not uncommon, but with both hands thus concealed, rare. A good deal depends upon the coat. The loose jacket-like garment worn by Mr. Stanford consorts well with the attitude. A frock coat would have been quite out of keeping. Photo graphers, unfortunately, have generally to deal with frock coats, and too often of the Sunday-go-to-meeting-store clothes variety. People sit to painters in their comfortable half-worn garments: why should they not thus sit to photo graphers? Mr. Herkomer apparently does not care to show much of his sitter’s hands. In No. 197 (Mr. William Sandbach) they are very subdued. One hand is resting on the other and conceals it, and only the knuckles of three
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