Management of Light in Illumination. 177 



It frequently happens, especially in large towns, that 

 rooms are so situated as to receive no light but what 

 comes through windows which open into narrow 

 streets or very small courts, and are so commanded 

 by high buildings as to receive very little light from 

 above. In all such cases, rooms would be much more 

 lighted and much better lighted by windows of ground 

 glass than by windows glazed with the finest trans- 

 parent glass. 



This I have found to be the case by experience, and 

 it may easily be explained. 



The rays of daylight which descend from the 

 heavens come down in a direction so nearly perpen- 

 dicular to the horizon that they impinge against the 

 polished surface of the glass so obliquely that most of 

 the rays are reflected in consequence of the smallness 

 of the angle of incidence ; and as those which enter 

 the glass and pass through it come into the room in 

 such a direction that they fall on the floor, where they 

 are mostly absorbed, they are of little use in lighting 

 the room ; but when the window is glazed with ground 

 glass, the surface of the glass which is rough being on 

 the outside, the asperities which the glass presents to the 

 descending rays greatly facilitate their entry into the 

 glass, and as in passing through it they are dispersed 

 in all directions the room will be much more equally 

 and more intensely illuminated than when the win- 

 dows are glazed with polished glass. 



The room in which the different classes of the 

 National Institute hold their ordinary weekly meetings 

 is surrounded by very high buildings on every side ; 

 and, its walls being covered with books quite up to the 

 ceiling, it was exceedingly dark and gloomy. All the 



