172 VITALITY AND EFFICIENCY WITH RESTRICTED DIET. 



accordance with the measurements and specifications given in his 

 article. We are also indebted to Dr. Percy W. Cobb for valuable sug- 

 gestions in reference to the instrument and its use. The two gentlemen 

 referred to can not, however, be held responsible for the form of instal- 

 lation which was adopted at the Nutrition Laboratory. 



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 separa- 

 tion between the lines ruled on the glass grating is -^^-^ 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 fig- 

 ure 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, desig- 

 nated 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 microm- 

 eter 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 microm- 

 eter 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 inter- 

 vening 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 mi- 

 crometer adjustment was gradually turned until the subject could see 

 fines, whereupon he called "stop" and gave the direction. If this 



