PSYCHOLOGICAL PROGRAM AND TECHNIQUE. 171 



found to be with an opening of 3 mm., which is much better than an 

 opening of 2 mm., but not significantly better than one of 4 mm. 

 This agrees thoroughly with the findings published by Cobb.^ 



The falling-plate camera, which, when in use, is at the right-hand end 

 of the hood (see H in the diagram in fig. 39), is so hinged that it may be 

 swung to the left and the visual-acuity object made to take its position 

 at the end of the hood. This arrangement is evident in figure 41. 

 Here the camera C has been swung partly out, but not far enough to 

 allow LH, the lamp house, carrying the visual-acuity object, to come 

 into position. The opening at the farther end of the hood may be seen 

 as a bright spot between the camera and the lamp house. 



The test object here used for determining the visual efficiency was 

 one in which the width of a set of alternately dark and light bands could 

 be continuously varied without changing in any way other factors in 

 the stimulus field. The average brightness of the field is the same for 

 all widths of lines. 



The principle upon which this type of test object is built, which 

 allows a continuous gradation in the width of the line without changing 

 the total light flux of the field, has been described in detail by Behn,^ 

 Ives,^ and Johnson.^ It is unnecessary to repeat details here. The 

 essential fact is this : If two glass plates are ruled with fine parallel lines, 

 the width of line and interspace being made the same, and the lines are 

 filled with an opaque substance, when these two plates are superim- 

 posed in such a position that their sets of lines are not quite parallel, 

 coarse feather-edged bands alternately light and dark and of equal 

 width will be seen distributed over the surface when light is transmitted 

 through both glasses. If one plate is slowly rotated in reference to the 

 other about an axis perpendicular to their surfaces, the width and total 

 number of visible lines will gradually change. The lines are always 

 uniformly distributed over the field of view (see figure 51, page 186). 

 As the angle of rotation is increased, each visible band, both light and 

 dark, decreases in width and new bands crowd into view from each 

 side of the field. A suitable mounting to hold two such ruled gratings 

 and to move them under measurable conditions slowly, and in oppo- 

 site directions about an axis perpendicular to their surfaces was 

 designed by Cobb.^ This mounting of Cobb, improved in certain par- 

 ticulars, is described by Johnson, who gives working drawings.*^ The 

 test object belonging to the Nutrition Laboratory was made for us at 

 the Nela Research Laboratory of the National Lamp Works, Cleve- 

 land, Ohio, under the kind direction of Dr. H. M. Johnson, and in 



^ Cobb, Am. Journ. Physiol., 1915, 36, p. 335. Data are given for three subjects. 



» Behn, Ber. d. deutsch. physikal. Gesellsch., 1906, 4, pp. 207 ff. 



s Ives, Electrical World, 1910, 40, p. 9-39. 



* Johnson, Journ. Animal Behavior, 1914, 4, p. 319. 



6 Cobb, Am. Journ. Physiol., 1911, 29, pp. 76 ff. 



^ Johnson, Journ. Animal Behavior, 1914, 4, p. 319. 



