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concave ; and they could also be made of curved and well-glazed 

 porcelain. 



Dead white surfaces of plaster of Paris and other materials have 

 been made for the purpose of obtaining a white light ; but, whether 

 flat, concave, or convex, they are only useful for very low powers, 

 in consequence of there not being a sufficient quantity of light ; and 

 this arises from its being radiated only from such surfaces in all 

 directions, but which I am able, with my mirror, to collect and bring 

 to a focus, along with those rays reflected from the surface of the 

 glass, thereby making use of the whole of the light that it is pos- 

 sible to obtain. 



That light is similarly affected by the colour of the surface it is 

 reflected from, I need only mention, as a familiar example, that when 

 a room is lighted only by a dull fire, the colour of the various articles 

 of furniture may be ascertained by that of the light they reflect on 

 the walls and ceiling, and which will show that the reflected light 

 from a white surface is white. 



