56 



INTENSITY OF LIGHT. 



It is represented in the engraving, where you see on the 

 left a screen similar to the one in the last figure, being 

 made of a plate of ground glass set in a frame. From the 

 centre of the plate there extends forward a shade or screen 

 of black pasteboard, which divides the ground glass plate 

 into two parts, and confines the light coming from each of 

 the two sources that are to be compared to its own side of 

 the screen. In the engraving, the light shining on one side 

 of the screen is seen to come from a jet of gas, while that 

 on the farther side is the light of a candle. On the table, 

 in lines extending from the glass to the lights, are scales 

 of inches, or other equal divisions, by which the distances 

 of the lights from the glass respectively can be at once 

 determined. 



Thus one half of the glass plate is illuminated by one 

 light, and the other by the other, and, by looking at the 

 two parts from the outer side, a very exact comparison 

 can be made between them. One or the other of the 

 lights must be moved until the two illuminations are pre- 



