IN ITS RELATION TO THE EYE. 447 



important factor especially if the light is not well diffused and the 

 surface of the object viewed is not sufficiently mat in character; but 

 not so important, for example, as a certain evenness or gradation of 

 surface brightness in the field of view. High brilliancies in fhe field 

 of view seem in fact to be the most important cause of the eye's 

 discomfort and loss in power to sustain clear seeing in lighting sys- 

 tems as we have them at the present time. In lighting from exposed 

 sources it is not infrequent to find the brightest surface from one 

 million to two and one half million times as brilHant as the darkest; 

 and from three hundred thousand to six hundred thousand times as 

 brilliant as the reading page. These extremes of brightness are, 

 our tests show, very fatiguing to the eye, especially when the high 

 brilliancies occur in certain zones or regions of the field of view. 



4. Of the commercial systems of artificial lighting tested thus 

 far, unmodified, the best results have been obtained for the indirect 

 systems, and the semi-indirect systems with reflectors having a high 

 density. By means of these reflectors the light is well distributed 

 in the field of view and extremes of surface brilliancy are kept 

 within the limits which the eyes are prepared to stand. A great 

 deal of loss in power to sustain clear seeing has been found to result 

 from the use of semi-indirect reflectors of low and medium density 

 and from the use of direct reflectors of shallow and medium depth. 

 With regard to the degree of density that is most favorable to the 

 eye, the direct reflector seems, however, to present a special case. 

 With translucent reflectors of medium depth, our best results have 

 been gotten so far with reflectors of medium density. This, how- 

 ever, is not in contradiction to our principle that extremes of bright- 

 ness are fatiguing to the eye. For if the physical efficiency of the 

 reflector is not to be lowered by increasing its density, its opening 

 must become brighter in some proportion to the increase of density; 

 i. e., in a totally opaque reflector all, and in the denser reflectors 

 nearly all of the light sent to the working plane must come from the 

 opening. Moreover, in case of the denser reflectors, the ceiling and 

 the reflectors are relatively dark, while standing out in sharp con- 

 trast to them is the bright opening of the reflector. In the reflectors 

 of medium density, however, the reflector need not have such a high 

 brilliancy and there is little contrast between it and its surroundings. 



