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EFFECTS OF UNIFORMITY AND CONTRAST ON LIGHT. 
By Hermann Bohle, F.R.S.S.Af. 
(Read August 17, 1910). 
In the planning of houses, halls, schools, and factories a good deal 
of sinning takes place as regards the lighting. The architectural features 
of the buildings receive preference. The questions of comfort and of 
adequate light are given second place ; and yet, for the welfare and 
comfort of the inhabitants, they are of vital importance. My remarks 
hold for day- and artificial light. Even in this country, with an almost 
eternal sunshine, one often finds rooms badly lighted even on the brightest 
days. Why? Too much daylight cannot be brought into a room. The 
human eye can adapt itself without harm to almost any daylight illumina- 
tion, as long as glare is avoided, and pass from a room with an illumination 
of forty candle-metres to an outside one of forty thousand without effort.* 
There is therefore no reason for making the inside illumination small ; 
it should certainly not be smaller than one-hundredth part of the 
outside one. 
Glare must, however, be avoided. An exact definition of glare has 
never been given. Some people define it as the intrinsic brilliancy,! 
which, when it exceeds a certain value — a value which depends on the 
individual — causes dazzling and pain to the eye. This definition of glare 
is not sufficient. If we look at the filament of an incandescent electric 
lamp in the evening we experience a dazzling sensation, or glare, but if we 
do so during the day the glare does not occur, or is at least greatly 
diminished. But the light is there all the same. Again, if we place a 
light in front of a black wall and look at it the glare appears strongly, 
whereas if it is placed before a white screen the glare vanishes. We see 
that intrinsic brilliancy alone does not constitute glare, and that contrast 
* A candle-metre denotes the illumination given by a standard candle on a perfectly 
white screen at a metre distance, assuming that all rays fall perpendicularly upon the 
screen. 
t Intrinsic brilliancy is the ratio of the normal intensity, measured in candles to the 
surface of the source. As regards the latter we should take the apparent source. 
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