14 Experiments 07i the Relative Intensities 



photometer, that the observer can turn it about, without 

 taking his eye from the field of the instrument. (See 

 Plate III., Fig. 3, and Plate IV., Fig. 4.) 



Many advantages are derived from this arrange- 

 ment : as, first, the observer can move the lights as he 

 finds necessary, without the help of an assistant, and 

 even without removing his eye from the shadows ; sec- 

 ondly, each light is always precisely in the line of 

 direction in which it ought to be, in order that the 

 shadows may be in contact in the middle of the verti- 

 cal plane of the photometer; and, thirdly, the sliding 

 motion of the lights being perfectly soft and gentle, 

 that motion produces little or no effect upon the lights 

 themselves, either to increase or diminish their bril- 

 liancy. 



These tables, which are 10 inches wide and 35 inches 

 high, and the one of them 12 feet and the other 

 20 feet long, are placed at an angle of 60° from each 

 other, and in such a situation with respect to the pho- 

 tometer that lines drawn through their middles in 

 the direction of their lengths meet in a point exactly 

 under the middle of the vertical plane or field of the 

 photometer, and from that point the distances of the 

 lights are measured; the sides of the tables being 

 divided into English inches, and a Vernier, showing 

 tenths of inches, being fixed to each of the sliding 

 carriages upon which the lights are placed. (See the 

 Plates III. and IV.) 



These carriages are so contrived that they can be 

 raised or lowered at pleasure, which is absolutely nec- 

 essary, in order that the lights may be always of a 

 proper height ; namely, that they may be in the same 

 horizontal plane with the tops of the cylinders of the 

 photometer. 



