322 ELEMENTS OF ELECTRICAL ENGINEERING. 



sun light, and all colors show up well by the light of such an arc 

 lamp. 



Glare. Excessive contrast of light and shade in the field of 

 vision tends to hinder visual perception. The eye adapts itself 

 automatically to the brightest lights in the field of view and all 

 perception of detail in the shadows is lost. This blotting out of 

 detail in the shadows by excessively brilliant lights in the field 

 of vision, which is called glare, is especially marked when the 

 field of vision includes a bright unshaded lamp. The explana- 

 tion of glare is as follows : In the first place, the pupil of the eye 

 contracts greatly when there is a bright light in the field of vision 

 and this contraction lessens the effective brightness not only of 

 the "high lights" but also of the deep shadows ; in the second 

 place the sensitiveness of the retina seems to be greatly reduced 

 in bright light, and, while this reduction of sensitiveness may 

 leave the eye able to perceive detail in the more brilliantly illumi- 

 nated portions of the field of vision it tends to obliterate all de- 

 tailed perception in the shadows ; and in the third place an 

 intense beam of light entering the eye from a bright source illu- 

 minates the whole interior of the eye, just as a beam of sunlight 

 entering a window illuminates a room, and this diffused light in the 

 eye illuminates and excites the portions of the retina where the 

 images of the shadows fall, and thereby obliterates all detail of 

 perception. 



An interesting case involving excessive contrast is that in 

 which a workman at a loom, for example, has his immediate 

 work illuminated to a fair degree of brightness while the remain- 

 der of the room is left in darkness. If the workman could keep 

 his eyes fixed upon his work incessantly, it is conceivable that 

 this kind of illumination might be satisfactory ; but the eye moves 

 about in spite of everything one can do, and under the assumed 

 conditions the workman would be unable to see when he glanced 

 about the room and he would be blinded when he glanced at his 

 work. To avoid this impracticable situation a general illumina- 

 tion of the room is necessary. 



