Bulletin of the Bureau of Standards Volume 5

Bulletin of the Bureau of Standards Volume 5

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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. 1909 edition. Excerpt: ...Wien Sitz., Ila, 116, pp. 1-24; 1906. HI. SUBJECTIVE LIGHT. 12. PHOTOMETRIC SENSIBILITY. Visual sensibility to variations in intensity decreases continually with increasing intensity. Compared with other physical instruments the eye has a most extraordinary range. It may be used with ease at intensities a million times the minimum perceptible. Its great power of accommodation is due chiefly to the decrease of sensibility with intensity. It is like an ammeter or galvanometer with an automatic shunt continuously variable. As a measure of relative sensibility the best data theoretically would be the mean errors in a long series of determinations of given fixed intensities in various parts of the spectrum. This has never been done and would be prohibitively tedious. Next best is probably the determinations of the least perceptible increment or "difference limen" by Konig. These include six different wave lengths and about twenty intensities of each. The least perceptible increment is greater than the mean error of observations of equality but proportional to it, although the exact relation between them is still undetermined. Konig referred all his intensities to a single large unit (about 600 meter candles) for all six wave lengths, and then at each wave length stepped down to intensities of 1/2, 1/5, 1/10, etc., of the large standard value. We shall refer to his intensities by the letter /, the threshold value being I, measured as a fraction of that high value. But although, on this basis, an illumination of 600 meter-candles in the red is of the same apparent brightness as 600 m-c. in the blue, 1 m-c. in the red is by no means of the same apparent brightness as 1 m-c. in the blue. At lower intensities the divergence is still greater....show more

Product details

  • Paperback | 126 pages
  • 189 x 246 x 7mm | 240g
  • Rarebooksclub.com
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236555899
  • 9781236555892