. Carnegie Institution of Washington publication. STUDIES ON SPECTRAL SENSITIVITY OF BIRDS. 89 the drawing, is the collimator, P! the direct vision prism, C^ the objective, and S3 the final slit. The total reflection prism TR throws the beam vertically down on the plaster-paris surface (stimulus patch 3 cm. X 5 cm.), which is placed in the compartment in which the response of the animal is obtained. Issuing from this second spectrometer system is a band of monochromatic light, which, when examined with a pocket spectrometer, shows no further resolution. In determining the curve of the relative
. Carnegie Institution of Washington publication. STUDIES ON SPECTRAL SENSITIVITY OF BIRDS. 89 the drawing, is the collimator, P! the direct vision prism, C^ the objective, and S3 the final slit. The total reflection prism TR throws the beam vertically down on the plaster-paris surface (stimulus patch 3 cm. X 5 cm.), which is placed in the compartment in which the response of the animal is obtained. Issuing from this second spectrometer system is a band of monochromatic light, which, when examined with a pocket spectrometer, shows no further resolution. In determining the curve of the relative stimulating value of light of differ- ent wave-lengths the bands of monochromatic light were equated in energy. Since thresholds were involved, the calibrated selenium cell, by means of which the energy was determined, was put 10 cm. above the stimulus patch and directly in the path of the beam. A. H. Pfund's method of calibrating with this cell will be found in Yerkes and Watson.* Even calibration before diffusion did not give a beam sufficiently low in intensity for direct use in threshold work. It was found necessary to introduce a wedge of smoked glass behind the final selecting slit. In the yellow-green, green, and blue regions a very dark glass was required; in the yellow and orange regions one very much. FIG. 9.—Control Box. H, home box; C, response chamber; F, FI, food compartments; X, Xi, stimulus patches; Si, S2, platforms causing lights L, L, to wink; S, platform causing both lights to wink simul- taneously; D, DI, doors leading to food compartments; K, KI, knobs controlling these doors. Sw lighter was required. Finally, in the red region no glass at all was required. Since these glasses were inserted after calibration, it was necessary to send them to the Bureau of Standards for calibration as to absolute absorption at the different wave-lengths employed in this work. The curve (fig. 10, p. 98) contains the proper corrections. When the appropriate smoked glass wa
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