Scientific Papers; 1887-1892 Volume 3

Scientific Papers; 1887-1892 Volume 3

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1902 edition. Excerpt: ...could in effect be by the use of a burning-glass throwing a solar image upon the slit. The employment of a telescope in the formation of the spectrum gives no means of escape from this conclusion. The precise definition of the brightness of any part of the resulting spectrum would give opportunity for a good deal of discussion; but for the present purpose it may suffice to suppose that, if the spectrum is to be divided into n distinguishable parts, so that its angular width is n times the angular width of the slit, the apparent brightness is of order l/n as compared with that of the sun. Under the conditions above supposed the angular width of the slit is in excess of the ocular limit, and the distance might be increased beyond d without prejudice to the brilliancy of the spectrum. As the angular width decreases, so does the angular dispersion necessary to attain a given degree of purity. But this process must not be continued to the point where w/d approaches the ocular limit. Beyond that limit it is evident that no accession of purity would attend an increase in d under given dispersion. Accordingly the dispersion could not be reduced, if the purity is to be maintained; and the brightness necessarily suffers. It must always be a condition of full brightness that the angular width of the slit exceed the ocular limit. Let us now suppose, on the other hand, that w2 is so small that the image of the sun is dilated to many times s, or that w2 is much less than X/s. The divergence of the light is now not s, but jw; and, if the pupil be just immersed, p/d = /w.2. The angular width of the slit w2/d is thus equal to /p, that is, it coincides with the ocular limit. The resulting spectrum necessarily falls short of full brightness, for it is evident that...
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Product details

  • Paperback | 208 pages
  • 189 x 246 x 11mm | 381g
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236560884
  • 9781236560889