84 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The loss ill visibility by the presence of a brilliant radiant in the 

 field of view is increased by the change in adaptation of the eye. It 

 is also probable that the intrinsic brilliancy of the radiant, as well as 

 the light received from it, has a bearing on the pupillary aperture. 

 Certainly at equal illuminations a well-shaded lamp gives higher visi- 

 bility than a bare one, both being assumed to be in the field of view. 

 There is therefore every reason for keeping such things as bare gas 

 lights and electric lamps entirely out of the visual field, only admitting 

 them thereto when they are so shaded as to keep the intrinsic brilliancy 

 to low limits. 



The eye has been evolved under conditions that imply rather 

 moderate intrinsic brilliancy, admitting the general desire to keep the 

 direct rays of the sun out of one's eyes. Sky light, of course, varies 

 very widely in apparent intensity, being most intense in the presence 

 of white cloud of moderate density. An average all the year round 

 mean for the northern part of the United States, giving the intrinsic 

 brilliancy of an aperture fully exposed to the upper sky, would be from 

 measurements by Dr. Basquin,^ in the neighborhood of 0.4 candle power 

 per square centimeter. This is lower than the intrinsic brilliancy of any 

 flame, and approximates that of a bright lamp behind a thin opal shade. 

 The ordinary window, which is in a wall rather than the roof, and gets 

 its light largely from low altitudes and somewhat reduced by trees or 

 buildings, is much less brilliant. 



For instance, a window 1 m. wide and 2 m. high would be unusually 

 effective if it gave 50 meter-candles at a point 5 m. within the room. 

 This illumination would imply a virtual intensity of about 1250 candles 

 at the window or an intrinsic brilliancy over the window area of 0.0625 

 candle power per square centimeter. Natural intrinsic brilliancies are 

 decidedly low, and the chief difference between natural and artificial 

 illumination, from the standpoint of wear and tear upon the visual 

 organs, is the high intrinsic brilliancy of artificial light. If radiants are 

 to be within the field of vision, they should be screened by diffusing 

 globes or shades down to a maximum intrinsic brilliancy of preferably 

 not above 0.1 or 0.2 candle power per square centimeter, certainly 

 not above double these figures. As I have pointed out in a former 

 paper,7 if one plots the pupillary apertures as ordinates and the 



function —= as abscissae, the result is nearly a straight line, so 

 that if one measures the visual usefulness u of a certain illumination 



8 The Illuminating Engineer, Jan., 1907. 

 ' Trans. 111. Eng. Soc., July, 1906. 



