342 H. M. JOHNSON 



foveas simultaneously, without much convergence. Objects 

 placed laterally to the bird may be imaged on the nasal fovea 

 and are often, if not usually, regarded monocularly. The chick 

 is thus not fairly representative of birds as a class. Its eye 

 shows convergence, and pupillary changes under conditions of 

 varying illumination and varying accommodation are marked. 

 Movements of rotation except for convergence are slight, how- 

 ever, and seldom made. 



As has been mentioned, the two test-fields for this problem 

 were both striate, in the horizontal direction. On the standard 

 field the individual striae were about 0.11 mm. wide, and it was 

 assumed that at the distance given they were too small to be 

 resolved by the eye. On the variable field, which was inter- 

 changeable with the standard field, the striae were larger, the 

 actual size in millimeters for each day of the experimentation 

 being shown in the accompanying tables. The test-fields were 

 prepared by using two pairs of superposed gratings, mounted as 

 described in the paper by the present writer cited above. Each 

 field was limited by a circular window 6 cm. in diameter, beveled 

 in an acid-blackened brass plate, which was placed behind the 

 window of the Yerkes experiment-box, with the test-object 

 close behind it. The test-objects were mounted at the windows 

 in the end of a two-compartment photometer-box, essentially 

 like that described by Yerkes and Watson. 3 Behind each test 

 object, in the compartment belonging to it, was mounted a 60 

 watt Mazda lamp, whose carriage rested on the photometer 

 track. Between each lamp and the test-object which it illuminated 

 was placed a diffusing screen of opal flashed glass, protected from 

 reflected light by diaphragms, and fixed to the track close behind 

 the test-object. The lamps were connected in multiple and 

 burned at normal voltage, the current being taken from a system 

 of 25-ampere storage cells. A voltmeter and rheostat connected 

 in series with the lamps enabled the experimenter to keep the 

 current constant. The test-fields were equated in brightness by 

 placing the lamps; their brightness was determined by the use 

 of a Sharp-Millar photometer, as 12.24 candles per square meter. 

 Considered as sources each field had a luminous intensity of 

 0.034 candles in the direction normal to its surface. The bright- 



1 Yerkes, R. M., and Watson, John B., " Methods of Testing Vision in Animals," 

 Behavior Monographs, vol. 1, No. 2, 1911. 



