Managcmeitt of Light in Illumination. 189 



such new inventions as may be useful in prosecuting 

 experimental inquiries. 



If I have ventured to place illumination among the 

 useful arts, if I have taken pains to investigate its 

 scientific principles, and to contrive instruments for 

 facilitating those inquiries which are still necessary in 

 order to carry it nearer to perfection, I am very far 

 indeed from supposing that it will be in my power 

 to finish that great and important work. 



I shall have done much if I succeed in turninof the 

 attention of ingenious men to this interesting subject ; 

 and I sincerely hope that the improvements resulting 

 from their united efforts will soon cause all those I 

 have proposed to be forgotten. 



As all improvements in illuminators must depend in 

 a great measure on the improvement of the methods 

 employed in the dispersion of light, and the choice of 

 the materials used for constructing luminous screens, it 

 may be of use to enlarge a little on that particular subject. 

 By constructing screens of different substances, but 

 of the same form and dimensions, and employing them 

 in pairs to mask the flames of lamps, which are made 

 to burn in such a manner as to emit equal quantities 

 of light, the relative quantities of light diffused by 

 those screens may easily be determined by means of 

 the photometer, and consequently the precise amount 

 of the loss of light which each of them occasions ; and 

 by a series of experiments of this kind, made with 

 screens composed of various substances, every thing 

 can be discovered that is necessary to be known, in 

 order to contrive the most efficacious means of dispers- 

 ing the too powerful light of the flames of lamps and 

 candles in the most agreeable manner and with the 

 least loss. 



