of the Light emitted by Luininous Bodies. 47 



is sitting before the photometer and looking at the 

 shadows, are both represented in this figure. 



Care must always be taken, in making the experi- 

 ments, to place the two lights and the centre of the 

 field of the photometer in the same plane. 



The frames of the long tables are constructed of 

 strong deal boards placed edgeways, and the two long 

 boards which form the two sides of each table are made 

 narrower at that end of them which is next to the stand 

 of the photometer, in the manner represented in the 

 Fig. 4. 



This is done to give more room to the observer, 

 when he is sitting: before the instrument to observe 

 the shadows. The winches are so placed that he can 

 conveniently keep one of them in each hand, and turn 

 them about while his eye remains fixed on the field of 

 the instrument. 



In order that the weight w, by which the cord is 

 kept properly stretched, may be forced to remain in 

 its proper place, the cord is made to pass over two 

 additional pulleys at a and b. The manner in which 

 these pulleys act will be evident from a bare inspection 

 of the figure. 



The upper edges of the two long boards which con- 

 stitute the insides of the frames of the two long tables 

 are divided in feet and inches, which greatly facilitates 

 the ascertaining of the distances of the lights from the 

 field of the photometer. 



At the ends of the long tables the pins are seen by 

 means of which the wires are stretched on which the 

 carriages of the lights slide. 



[This paper is printed from Rumford's Philosophical Papers, Vol. I., 

 pp. 270-318.] 



