466 H. M. JOHNSON 



the correctness or incorrectness of the reaction furnished an 

 objective check on the subject's mode of response, also influenced 

 the choice of this type of determination. 



APPARATUS AND PROCEDURE 



In the present work the conditions of Cobb's studies were 

 imitated as far as was practicable. A photometric field, in 

 sensibly perfect balance, is viewed monocularly 7 by the subject 

 from E (fig. 1) through the limiting rectangular window W of 

 a hollow polyhedron (7, the interior surfaces of which fill the 

 remainder of the visual field. The brightnesses of the latter 

 are practically unity with respect to each other, but may be 

 varied at will with respect to the brightness of the photometric 

 field, the two illuminations being almost completely independent. 



The photometric field is formed as follows : A milk-glasssurface 

 MG 3 , illuminated by a tungsten incandescent lamp L 3 , is viewed 

 directly by the subject through a 10 degree double prism P. 

 Two similar surfaces, MGi and MG 2 , illuminated by lamps Li 

 and Z/2, respectively, are viewed by reflection from the silvered 

 mirrors M l and M z respectively, and by partial reflection from 

 the front surfaces of P. The silvered mirrors are used on account 

 of the limited area of P and the limited distance WP. If Li 

 and Z/2 are properly placed, the result is a balanced photometric 

 field projected in the plane of P, 150 cm. from the eye, and 

 limited by the dimensions of W. These, in terms of visual angle 

 subtended by them, are approximately 1.96 degrees vertically 

 and 2.65 degrees horizontally; so that when properly fixated, the 

 image of the test field covers the fovea and extends but little 

 beyond it. The distribution of intensities among the wave- 

 lengths of the visible spectrum is approximately that of a black 

 body at 2400K. 



The stimulus to reaction is the darkening of one-half of the 

 photometric field to the extent of the brightness added by Li 



7 The unused eye is covered by a sheet of ground glass, through which light 

 from the interior of C is diffusely transmitted with little reduction by absorption. 

 This tends to stabilize the relative adaptation of the two retinas, and reduces the 

 pupillary fluctuation which might otherwise result. 



