photographic definition from light of different wave-lengths

3
Photographic Definition from Light of Different Wave-Lengths Author(s): C. D. Perrine Source: Publications of the Astronomical Society of the Pacific, Vol. 20, No. 123 (December 10, 1908), pp. 292-293 Published by: The University of Chicago Press on behalf of the Astronomical Society of the Pacific Stable URL: http://www.jstor.org/stable/40692978 . Accessed: 23/05/2014 14:40 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . The University of Chicago Press and Astronomical Society of the Pacific are collaborating with JSTOR to digitize, preserve and extend access to Publications of the Astronomical Society of the Pacific. http://www.jstor.org This content downloaded from 194.29.185.70 on Fri, 23 May 2014 14:40:46 PM All use subject to JSTOR Terms and Conditions

Upload: c-d-perrine

Post on 10-Jan-2017

213 views

Category:

Documents


0 download

TRANSCRIPT

Photographic Definition from Light of Different Wave-LengthsAuthor(s): C. D. PerrineSource: Publications of the Astronomical Society of the Pacific, Vol. 20, No. 123 (December 10,1908), pp. 292-293Published by: The University of Chicago Press on behalf of the Astronomical Society of the PacificStable URL: http://www.jstor.org/stable/40692978 .

Accessed: 23/05/2014 14:40

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

The University of Chicago Press and Astronomical Society of the Pacific are collaborating with JSTOR todigitize, preserve and extend access to Publications of the Astronomical Society of the Pacific.

http://www.jstor.org

This content downloaded from 194.29.185.70 on Fri, 23 May 2014 14:40:46 PMAll use subject to JSTOR Terms and Conditions

292 Publications of the

years, to determine the positions and the proper motions of the stars. This plan, embracing every star in the entire sky down to the seventh magnitude, is one of the most extensive and ambitious recorded in the history of astronomy; and there is full confidence that the results will be correspondingly valuable.

The site for the observing station was fixed at a point about a kilometer from the center of the city of San Luis, in the west-central part of Argentina. Professor Boss has written that the government officials and the people have been most sympathetic with his aims. The site was provided by the National Government on national property, and in many other ways definite assistance was afforded. Building operations were well under way within two weeks after the astron- omers landed in Buenos Aires. Professor Tucker will be astronomer in charge of the observing station during the three years of work planned for. He will have a large corps of assistants in order that the instrument may be kept busy and the calculations be made promptly.

Albany newspapers report that the ship on which Professor Boss took homeward passage was wrecked shortly after leav- ing Buenos Aires, and if the reports are trustworthy there must have been cause for great anxiety for a day or two.

W. W. Campbell.

Photographic Definition from Light of Different Wave-Lengths.

The large size of the photographic images of stars obtained with telescopes of sixty to seventy feet focal length, has led the writer to look for means of reducing them.

Without going into the matter of "seeing" deeply, it may be said that, in all probability, the character of the images depends almost wholly upon atmospheric refraction. Assum- ing this to be the case, star images, for example, formed by yellow light should be much less affected by disturbances of the atmosphere, and consequently be smaller than images formed by light of the short wave-lengths (blue and violet), which affect the ordinary photographic plate most.

This content downloaded from 194.29.185.70 on Fri, 23 May 2014 14:40:46 PMAll use subject to JSTOR Terms and Conditions

Astronomical Society of the Pacific. 293

Photographs taken with the new 70-foot combination of the Crossley reflector, in the ordinary blue and violet light, and also on orthochromatic plates, using a greenish-yellow color screen, show that the star images which have been formed by yellow light are much smaller than those formed by the ordinary blue and violet light. Similarly, photographs of the Moon and planets are sharper in yellow than in blue light.

Extension of this reasoning points to the deep red part of the spectrum as being favorable for the production of still smaller images and better definition. In photographic work on bright objects, such as the Sun, Moon, and the brighter stars and planets, the use of light of great wave-length is entirely practicable.

A consideration of visual definition seems to point to an analogous effect there, although to a less degree, owing to the greater wave-lengths to which the eye is most sensitive.

A detailed account of the experiments already concluded, as well as some still in progress, will be published shortly in a Lick Observatory Bulletin. C. D. Perrine.

Mt. Hamilton, Cal., December 5, 1908.

Eighteen Stars Whose Radial Velocities Vary. In the progress of the work with the three-prism Mills

spectrograph at Mt. Hamilton, and of the D. O. Mills Expedi- tion to the southern hemisphere, eighteen stars have recently been found to have radial velocities which vary, on account of the gravitational influence of an invisible but massive com- panion in each system, except that in case of one star, 70 Ophiuchi, a well-known visual double star, the companion is visible. A list of these stars, with data concerning their positions, number of observations, total range of observed velocities, and discoverers of the variations, is given below. Only in the case of yo. Ophiuchi is the revolution period known, - in that case eighty-eight years. Detailed data con- cerning the observations are contained in a Lick Observatory Bulletin now in press.

This content downloaded from 194.29.185.70 on Fri, 23 May 2014 14:40:46 PMAll use subject to JSTOR Terms and Conditions