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  1. The authors are pleased to acknowledge the advice and assistance of Mr. E. G. Lapham of the Bureau's radio section in connection with their study of the electrical parts of the instrument.
  2. The variable capacity was suggested by Mr. Michaelson of the General Electric Company to whom the authors are greatly indebted for the care with which the instrument was installed and tested, not only at the original installation but upon the occasion of a return visit about a year later.
  3. Measurements of absolute reflectance are, of course, possible with specially designed equipment and such measurements have often been made as a function of wavelength in the case of polished metal surfaces. Commercial spectrophotometers are not designed primarily for such measurements; an approximation to absolute reflectance values may be obtained on the G. E. instrument, however, if (1) the MgO comparison surface is fresh, (2) the interior of the sphere is uniformly coated with MgO, and (3) the polished surface is inclined so that all the specularly reflected light is directed into the sphere, as was done in Fig. 4.
  4. H. J. McNicholas, Nat. Bur. Stand. J. Research 1, 29 (1928) RP 3.
  5. The identity of two samples can be tested by making two measurements with the samples interchanged between the measurements. Identical samples will give identical curves.
  6. The difference in the two curves in the region of 570 mµA is not a slit-width effect. The upper curve has lagged; this is due to a certain "blocking effect" often caused on this very steep curve if the screen-grid voltage is too high. No such blocking occurred, for example, in the didymium curves of Figs. 2 and 3.
  7. The authors are pleased to acknowledge the assistance of Mrs. G. W. Haupt in the determination of these values, also in certain of the values given in Tables II and III, below.
  8. Kasson S. Gibson, Nat. Bur. Stand. J. Research 14, 545 (1935) RP 785; also J. Opt. Soc. Am. 25, 131 (1935).
  9. A further check on the accuracy of the wave-length scale is to let the instrument plot a vertical line across the whole sheet, as has been recommended by Mr. Pineo. This line must be accurately parallel to the vertical lines of the network. One should be satisfied, by test or experience, that this condition is fulfilled on each sheet.
  10. The λet curve of Fig. 2 will be considered later.
  11. "Preparation and Colorimetric Properties of a Magnesium-Oxide Reflectance Standard," NBS Letter Circular, LC-395, 1936.
  12. K. S. Gibson, G. W. Walker, M. E. Brown, J. Opt .Soc. Am. 24, 58 (1934).
  13. See paper by Mr. Michaelson, preceding this paper.
  14. These results were presented at the Color-Conference held at M. I. T. in July, 1936.
  15. J. Opt. Soc. Am. 28, 180 (1938).

Brown, M. E.

K. S. Gibson, G. W. Walker, M. E. Brown, J. Opt .Soc. Am. 24, 58 (1934).

Gibson, K. S.

K. S. Gibson, G. W. Walker, M. E. Brown, J. Opt .Soc. Am. 24, 58 (1934).

Gibson, Kasson S.

Kasson S. Gibson, Nat. Bur. Stand. J. Research 14, 545 (1935) RP 785; also J. Opt. Soc. Am. 25, 131 (1935).

Haupt, G. W.

The authors are pleased to acknowledge the assistance of Mrs. G. W. Haupt in the determination of these values, also in certain of the values given in Tables II and III, below.

McNicholas, H. J.

H. J. McNicholas, Nat. Bur. Stand. J. Research 1, 29 (1928) RP 3.

Walker, G. W.

K. S. Gibson, G. W. Walker, M. E. Brown, J. Opt .Soc. Am. 24, 58 (1934).

Other (15)

The authors are pleased to acknowledge the advice and assistance of Mr. E. G. Lapham of the Bureau's radio section in connection with their study of the electrical parts of the instrument.

The variable capacity was suggested by Mr. Michaelson of the General Electric Company to whom the authors are greatly indebted for the care with which the instrument was installed and tested, not only at the original installation but upon the occasion of a return visit about a year later.

Measurements of absolute reflectance are, of course, possible with specially designed equipment and such measurements have often been made as a function of wavelength in the case of polished metal surfaces. Commercial spectrophotometers are not designed primarily for such measurements; an approximation to absolute reflectance values may be obtained on the G. E. instrument, however, if (1) the MgO comparison surface is fresh, (2) the interior of the sphere is uniformly coated with MgO, and (3) the polished surface is inclined so that all the specularly reflected light is directed into the sphere, as was done in Fig. 4.

H. J. McNicholas, Nat. Bur. Stand. J. Research 1, 29 (1928) RP 3.

The identity of two samples can be tested by making two measurements with the samples interchanged between the measurements. Identical samples will give identical curves.

The difference in the two curves in the region of 570 mµA is not a slit-width effect. The upper curve has lagged; this is due to a certain "blocking effect" often caused on this very steep curve if the screen-grid voltage is too high. No such blocking occurred, for example, in the didymium curves of Figs. 2 and 3.

The authors are pleased to acknowledge the assistance of Mrs. G. W. Haupt in the determination of these values, also in certain of the values given in Tables II and III, below.

Kasson S. Gibson, Nat. Bur. Stand. J. Research 14, 545 (1935) RP 785; also J. Opt. Soc. Am. 25, 131 (1935).

A further check on the accuracy of the wave-length scale is to let the instrument plot a vertical line across the whole sheet, as has been recommended by Mr. Pineo. This line must be accurately parallel to the vertical lines of the network. One should be satisfied, by test or experience, that this condition is fulfilled on each sheet.

The λet curve of Fig. 2 will be considered later.

"Preparation and Colorimetric Properties of a Magnesium-Oxide Reflectance Standard," NBS Letter Circular, LC-395, 1936.

K. S. Gibson, G. W. Walker, M. E. Brown, J. Opt .Soc. Am. 24, 58 (1934).

See paper by Mr. Michaelson, preceding this paper.

These results were presented at the Color-Conference held at M. I. T. in July, 1936.

J. Opt. Soc. Am. 28, 180 (1938).

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