Management of Light in Illumination. 109 



planes of those infinitely small portions of the broken 

 surface of the glass where the rays happen to pass. 



If the flame of a burning candle be placed in the 

 centre of a large globe of very fine transparent glass, 

 its rays will pass through the glass without suffering 

 any sensible alteration, either in their direction or in 

 their intensity; and the form and dimensions of the 

 flame will be seen so distinctly through the glass that, 

 at a little distance, the globe might easily escape obser- 

 vation. But if, instead of placing the candle in a globe 

 of transparent glass, it be placed in the centre of a 

 globe of ground glass, the rays of light will be so dis- 

 persed in passing through it that from each visible 

 point of its external surface rays will be sent off in all 

 directions, which will render the surface of the globe 

 luminous. The flame of the candle will no longer be 

 seen through it, but surrounding bodies will not be 

 less illuminated on that account. 



The globe will be the only luminous body which 

 will be visible ; and as the intensity of the light at its 

 surface may be diminished without any loss, merely 

 by increasing that surface by augmenting the diame- 

 ter of the globe, it is evident that by a judicious ar- 

 rangement of screens of ground glass, or of other fit 

 substances, the too vivid light of lamps may be so dis- 

 persed and softened without any considerable loss as 

 to protect the eyes from injur)^, and at the same time 

 render the illumination infinitely more mild, tranquil, 

 and agreeable. 



But if screens can be found which do not sensibly 

 diminish the light employed to render them luminous, 

 and if their forms and dimensions can be varied with- 

 out inconvenience, there can be no longer any difficulty 



