domingo, 18 de outubro de 2009

15 de Agosto


One of the most interesting and valuable experiments was performed in Cambridge on the 28th ult., at the time of the eclipse of the sun. W. C. Bond, Director of the Cambridge Observatory, R. T. Paine and Mr. Tuttle, Assistants, Mr. S. C. Walker, of the Coast Survey, and J. A. Whipple, Daguerreotypist of Boston, were present at the Observatory. While the other gentlemen were engaged with different instruments, Mr. Whipple was prepared with a number of Daguerreotype plates to secure, if possible, the appearance of the sun at different times, as it was more or less obscured from view. We take the liberty of making a few extracts from a letter to us, written by Mr. Whipple.
"I have just been engaged in a very interesting experiment-taking Daguerreotypes of the sun. Although, at first thought, it appears so easy, yet, upon trial, I found it a very difficult matter to accomplish to my own, as well as to the satisfaction of others. One great difficulty was the excess of light which was in some way to be disposed of. In order to meet this difficulty, my first plan was to reduce the aperture; I found, however, that this could not be caused to the desired extent, and produce good results, on account of interference and inflection of the light destroying the definition of the image. When the aperture was large enough not to produce sensible inflection, the quickest possible motion of the hand was too long, although I did not use the most sensitive coating for the plate."
Mr. W. next resorted to mechanical means, but found even the same trouble, only in less degree-the impressions were little more marked; still the slide that passed over the opening of the lens caused a blur on one side; the opposite side was much better defined-thus, showing, conclusively, that this plan would not produce the desired result. The most flattering results were obtained by reducing the aperture as small as possible, and at the same time allowing a free and distinct image on the ground glass, also using the least sensitive coating. By these means, the Daguerreotypes taken answered Prof. Bond's purpose for measurement, &c.
The instrument used for producing these images was an object lens three inches in diameter; focal length, three feet four inches; eye piece, four inches; combined focal length, nine feet, giving an image of the sun, about four inches in diameter.
Mr. W. says-"We made ten impressions at different times, from its first appearance to the time of its greatest obscuration. It is to be regretted that gathering of clouds prevented any further attempts."
By knowing the exact instant of time these Daguerreotypes were taken there, and close measurement, important results can be arrived at with certainty. In speaking of the appearance on the ground glass, Mr. W. says-"The sun's image on the ground glass was most beautiful; its round disk floating majestically in the air, with a slight tremulous motion occasioned by the different strata of air, as he holds his onward course-a slight indentation on one edge-it increases, gradually assuming a circular form with rounded edges-(the elevation and depressions on the moon's surface,) still increasing, until nearly one-third of the surface had disappeared. Alas! like things of earth, much to our disappointment, clouds prohibit further observation."

Sem comentários:

Enviar um comentário