170 VITALITY AND EFFICIENCY WITH RESTRICTED DIET. 



that the artificial pupil was as close as convenient to the eye of the sub- 

 ject. The distance was usually not more than 1 cm. At the other end 

 of the tube there was a diaphragm (see D in fig. 39). The round open- 

 ing in this was 12 nun. in diameter. Its use and importance will 

 become clear in later paragraphs. 



. ^ 



Fig. 39. — Arrangement of apparatus for the measurement of visual efficiency. 



LH, lamp house; X, mirror to reflect light through the test object, 0; M, micrometer for adjust- 

 ing width of test bands in O; H, hood excluding extraneous light; S, velvet screens to reduce 

 reflected light from walls of hood; T, telescoping member at one end of which is the dia- 

 phragm, D, for limiting the area of view ; at the other end the artificial pupil, P, is placed near 

 the subject's eye; R, head-rest shown in figure 32. 



The artificial pupil was a round opening in a flat black surface and 

 was 3 mm. in diameter. This size of artificial pupil was adopted as the 

 result of a large series of observations which had been carried out previ- 



Fig. 40. — Visual efficiency with different diameters of artificial pupil. 

 The ordinate marked 5 mm.+ shows results when no artificial pupil was used. 



ous to the present research. Data for this factor are shown in the 

 curves of figure 40. These curves are based on about 1,200 threshold 

 determinations on one subject, who served on different days, but under 

 conditions which may be considered as uniform. Optimum vision is 



