490 H. M. JOHNSON 



themselves chiefly with eliminating extreme conditions of 

 "glare."" 



While the preliminary experiment was not planned to ascertain 

 what is the optimal distribution of brightnesses, certain definite 

 results were incidentally obtained. The author believes these 

 results to be susceptible of misinterpretation unless certain facts 

 are especially considered. It will be recalled that the area of 

 the stimulus was small, its image barely covering the fovea. 

 Under this condition the brightness of the surroundings deter- 

 mined the state of adaptation of the retina. In such case, 

 therefore, one may suspect that the brightness of the surround- 

 ings which best adapts the retina to the brightness of the stimulus 

 will always be the optimal condition. This has been borne out 

 by the work of Cobb and of Cobb and Geissler on the thresholds 

 for pattern and for difference of brightness, and has been cor- 

 roborated by the present work to the extent of the limited range 

 of distributions of brightnesses employed. 



Under ordinary lighting conditions, however, as in the home 

 or office, the page which one may be reading, or the sewing on 

 one's lap, may cover a large part of the visual field. The paper 

 on which this report was written subtended a visual angle of 

 40 degrees in one dimension and 60 degrees in the other, at the 

 distance from the eye at which it lay during the work. The 

 writer has tried several "uniform" distributions of illumination 

 in the home and office, and has discarded all of them in favor 



13 For the benefit of any student of illuminating engineering who may still 

 be puzzled by the significance of differences of a few thousandths of a second 

 in the time required for recognition, I have the kind permission of Capt. P. W. 

 Cobb to refer to a suggestion of his, contained in a paper which, I trust, will 

 soon be published. The suggestion is that the normal resting period of the eye 

 is but momentary; and that any condition which increases the time required for 

 perception, even by a small amount, may tend to retard the normal rate of eye- 

 movement and hence to induce excessive fatigue of the extrinsic muscles. The 

 case is somewhat analogous to the movements in walking. If one is compelled 

 to walk more slowly than is one's wont, or to adopt an irregular gait, the added 

 exertion is at once noticeable. It is often noted that the reading of unfamiliar 

 subject-matter is much more fatiguing than the reading of familiar matter for 

 the reason that the eye must rest for a longer period on the words or group of 

 words comprehended in a single observation. 



