Excerpt from Interference Measurements of Wave Lengths in the Iron Spectrum, 3233a-6750a

Seed 27 plates were used throughout, but to photograph the region 4900a to 67 50a the plates were'bathed in pinaverdol and dicyanin with ammonia. In the region 32'o0a to 4200a there were measured: Four exposures of 1; five of 6; one each of 15, 20, and 30; and two of 60' minutes' duration. In the region 3 500a to 4500a the exposures were: Five of 2; five of 5 two of 20; and three of 25 minutes. The region 4100a to 5100a was covered by exposures as follows: Three of 3; two of 5; one of 10; four of 15; one of 25; and four of 50 minutes. Finally, in the region 4900a to 6800a, the following exposures were measured: One of 1; two of 3; three of 5; three of 9; two of 15; three of 20; two of 90; and one each of 65, 80, and 100 minutes' duration. The first three regions were photographed in the second order; 4900a to 6800a was taken in the first order. In almost every case two or more exposures were measured on the same plate.

These exposures were made without disturbing the interferome ter. In 3 cases out of 24 there Was a change in the thickness of about one part in a million, and in a few other cases there was a barely perceptible change. These changes'were probably due to a variation in the temperature, although the thermometer beside the interferometer registered no appreciable change. By allow ing the light to fall upon the interferometer for half an hour before taking the plate this trouble could be avoided. A short exposure was always made first, and another short exposure usually followed the long one. In any case, each exposure was worked out by itself and this expansion of the interferometer caused no difficulty. In a few cases the long exposure on a plate was too dense to permit of accurate measurement of a sufficient number of standards. In such cases the long and short exposures were tied together-by lines of intermediate intensity. It will be seen at once that this method would lead to' erroneous results in case there were an intensity equation. The question will be discussed later in connection with the grating observations.

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