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,2 Ives,3 and Johnson.4 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 superimposed 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 opposite directions about an axis perpendicular to their surfaces was designed by Cobb.6 This mounting of Cobb, improved in certain particulars, is described by Johnson, who gives working drawings.6 The test object belonging to the Nutrition Laboratory was made for us at the Nela Research Laboratory of the National Lamp Works, Cleveland, Ohio, under the kind direction of Dr. H. M. Johnson, and in accordance with the measurements and specifications given in his article. We are also indebted to Dr. Percy W. Cobb for valuable suggestions in reference to the instrument and its use. The two gentlemen referred to can not, however, be held responsible for the form of installation which was adopted at the Nutrition Laboratory.

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

2 Behn, Ber. d. deutech. physikal. Gesellsch., 1906, 4, pp. 207 ff.

3 Ives, Electrical World, 1910, 40, p. 939.

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

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

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

Some details of the test object, together with its form of mounting and means of illumination, may be seen in figure 42. Faint vertical lines appear in the test field as shown in the photograph. As viewed by a subject who is being tested, the contrast between the light and dark lines is of course much sharper than in the illustration. The separation between the lines ruled on the glass grating is 1 240 inch, and the rulings, which are filled with an opaque substance, are the same width as the interspaces. The plates are held in metal rings, R and R' of figure 42. The metal rings are retained in such a position that they can be rotated on an axis perpendicular to their surface by three lugs, designated as L in the figure. The yoke Y is large enough to span the diameter of the metal rings which hold the gratings. One tip of the yoke has a bearing on R, while the other tip bears on R'. A micrometer adjustment M moves the yoke against the tension of the spring S. Thus, by turning the handle of the micrometer, the two gratings are caused to rotate slowly in reference to each other. Each grating is moved, but in opposite directions. The base which carries the micrometer adjustment, the yoke, and the metal rings for the glass gratings can be rotated so that the lines will appear in any axis desired. It can be accurately set to any axis, as degrees are marked on the scale D. When in the position desired, it is clamped by C. The test object is shown to be at the end of the hood H. The lamp house, which is open in figure 42, is white inside and contains one carbon filament lamp. The bottom B is covered with a mirror. Above the lamp are two thicknesses of ground glass G. In the upper part of the lamp house, along the surface X, is located a second mirror, which reflects the light to the milk-glass window W, which, when the instrument is in use, is directly behind the test field and illuminates it evenly. A soft leather washer mounted on the outer ring R of the test object fills up the intervening space between the window of the lamp house and the test object when in position, so that no indirect illumination interferes. The diameter of the window W is 10 cm.; that of the test field is slightly less than 9.5 cm.

The adaptability of the test object, which made it easily possible to present the lines in any desired axis, was a feature of merit. By working from invisibility to visibility, it was thus possible, in taking any threshold measurements on a subject, to check his observation, since he did not know at what axis the lines would appear. The micrometer adjustment was gradually turned until the subject could see lines, whereupon he called "stop" and gave the direction. If this corresponded to the position of the test object, it was reasonable proof that the subject had actually seen the lines and had not reacted to some false impression, which, with many untrained subjects, would quite naturally be the case, provided the subject knew in what axis the lines were going to appear. The test object window was round so that a shift in the axis would produce no observable difference in its shape or position. Experience has shown, however, that an experimental difficulty is encountered when the whole window and frame are exposed to the subject.

In figure 51 (see page 186), we reproduce a full-sized photograph of the window of the test object. The bottom and right-hand edges of the figure, as well as the lower right-hand portion, have been covered with a black paper mat; the other portion shows the circular boundary of the window. If the reader will hold this illustration at arm's length it will be seen that certain parts of the boundary for the light and dark lines have particular prominence, as, for example, positions 1 and 3, as contrasted with 2 and 4. This phenomenon can not be seen in the illustrations which have been shown by Ives,1 who figures only square areas of such striae as are under consideration. When looking at the test field, surrounded by a definite black circular frame, it is possible, after a little practice, to sense the presence of lines and to give their direction on the basis of this intersection phenomenon where the lines form acute angles with the dark border. It is also possible to do this with smaller lines than can be seen in the center of the field. When using the test object in only one axis, as has apparently been the custom at Nela Research Laboratory, where Ives, Cobb, Johnson, Luckiesh, and others have used this test object in a number of researches, it is doubtless possible with practised subjects to instruct them to give attention only to the center of the field, and judge when lines appear there.