PRODUCTS FROM COAL. 



41 



lights shine on the same surface at equal obliquities, and an opake body be 

 interposed between each of them and the illuminated surface, the two shadows 

 must differ in intensity and blackness, for the shadow formed by intercepting 

 the greater light will be illuminated by the lesser light only ; and inversely, 

 the other shadow will be illuminated by the greater light, that is, the stronger 

 light will be attended with the deeper shadow : but it is easy, by removing 

 the stronger light to a greater distance, to render the shadow which it pro- 

 duces not deeper than that of the smaller, or of precisely the same intensity ; 

 this equalization being effected, the quantity of light emitted by each lamp 

 or candle will be as the square of the distance of the burning body from the 

 illuminated surface. By reference to the diagram (Fig. 8.) the instrument will, 

 I think, be rendered perfectly intelligible. 



Fig. 8. 



A is a wooden box, painted black inside, except at the back B, which is 

 painted white (or a sheet of fine white paper fastened to it may be prefer- 



