STUDIES ON SPECTRAL SENSITIVITY OF BIRDS. 



89 



the drawing, is the collimator, Pj the direct vision prism, Oi 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 directl}' 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, S^. 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. 



lighter was required. Finallj-, 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 was 

 inserted the threshold for the animal was found to lie well within the range of 

 intensities offered by the Brodhun sector. 



The animal's reactions were taken in the apparatus shown in figure 9, which 

 consists of a large box, 94.5 cm. in length by 74 cm. in \\ndth by 2.5 cm. in 

 depth. It is divided into a home compartment H, 31.5 cm. in length by 25 

 cm. in width, and a response chamber C, 30.5 cm. in width, which is partially 

 separated into two smaller compartments by a partition. The length of the 

 partition is 25.5 cm. The distance from the door leading into the response 

 chamber to the stimulus patch is 69 cm. The distance between the stimulus 

 patches is 25 cm. The dimensions of the face of the stimulus patch are 3 cm. 

 by 5 cm. The other divisions of the box are not important. 



♦Methods of Studying Vision in Animals, Behav. Mono., 2. 



