174 VITALITY AND EFFICIENCY WITH RESTRICTED DIET. 



gible quantity any disturbance from the phenomenon illustrated in 

 figure 51, but with the definite boundary of the test field hidden from 

 view, and in its place a hazy fading-out of the lines on all sides, the 

 subject found difficulty in keeping the eye accommodated to the proper 

 distance when lines were not in view, as just preceding the taking of 

 threshold measurements. It was found possible to fasten a small disk 

 of black paper 2 nun. in diameter to the surface of the glass grating 

 which was nearer to the subject. This provided a fixation point for 

 accommodation before the lines of the field came into view. The cen- 

 tral line of the field directly behind the fixation point changed only in 

 width, and a few lines on either side had but slight lateral move- 

 ments. This position was, therefore, very favorable for fixation. The 

 artificial pupil performed an important service in making a fixed view- 

 ing-point for the subject. Had it been larger, or had there been no 

 artificial pupil, it would have been possible for him, by moving to one 

 side or the other, to have exposed the definite, circular frame of the 

 test field. With a 3-mm. opening of the artificial pupil, it was impossi- 

 ble to get in any position with the apparatus as used, from which the 

 definite edge of the test field could be seen, and when the subject 

 moved slightly one way or the other, he could immediately recognize 

 the dimming of the field and that he must be out of position. 



The luminant was an 8 c. p. carbon filament lamp. The brilliancy of 

 the test-object window in candles per square meter of surface was 20.3. 

 With a 16 c. p. carbon lamp, the brilliancy in candles per square meter 

 was 57.8. In Cobb's apparatus the diameter of the window exposed to 

 view was 3.5 cm. The brilliancy of the source in candles per square 

 meter he varied from 5.94 to 189.0. Increased intensity of illumination 

 beyond a certain point does not greatly aid the eye in distinguishing 

 detail. Fig. 40 (p. 170), which gives results with 8 c. p. and 16 c. p. 

 lamps, shows that although the latter supplied an illumination about 

 three times that with the 8 c. p. lamp, yet the visual efficiency was 

 not greatly improved. As the brighter light was much more tiresome 

 to the eye, the 8 c. p. lamp was used in illuminating the test object 

 for these experiments. 



The first time a subject was given this test at the Laboratory he was 

 instructed as follows : 



"You will now look through this small peep-sight with the right eye and 

 see a light window about 6 feet away from you. In the window are dark 

 bands. Please notice that they may be made small or large as we desire. 

 We want to discover the size of the smallest lines that you can see. Our 

 method is as follows : We will first make the lines so small that they disappear 

 from view; they will then be gradually widened until you can see them, 

 whereupon you will call 'stop.' Please notice, also, that the fines may be 

 vertical or horizontal or they may be in a diagonal (45°) position with their 

 tops to the right or to the left. The four positions will be used, viz, right, 

 vertical, left, and horizontal. You can never tell beforehand in what direction 



