46 Experiments on the Relative Intensities 



a proper degree of tension by a weight under the table 

 which is fixed to a pulle}^ This weight is seen in the 

 next figure (Plate IV.) suspended by the cord under 

 one of the tables. T is the circular table, which is rep- 

 resented on a much larger scale in the Figs, i and 2. 



In this figure (3) and in the next, the brackets are 

 seen which support the ends of the long tables which 

 are farthest from the stand of the photometer. Each 

 of these brackets is furnished with two screws, dis- 

 tinctly represented in the figure, which serve for set- 

 ting the table on a true horizontal level. 



Plate IV., Fig. 4. This figure is an elevation of the 

 whole of the machinery, seen in the direction of the 

 length of one of the long tables. The two tables are 

 supposed to be so placed as to form an angle of 60°, in 

 which situation they are also represented in the last 

 figure. As in this figure (4) one end of one of the 

 long tables is represented as standing immediately 

 before the stand of the photometer, the sliding car- 

 riaofe belongins^ to that table obstructs the view of the 

 upper part of the stand, and of the box of the photom- 

 eter, and renders the appearance of the machinery in 

 that part of the plate rather confused ; but by a care- 

 ful examination the different parts of it may be distin- 

 guished. 



The platforms on which the lights are placed are 

 represented as being both fixed at the same horizontal 

 level ; and all the six hollow knobs of wood are dis- 

 tinctly seen, by which they are fastened to the slender 

 pillars which support them. The lights themselves 

 are not represented in any of these figures. The 

 handles of the winches, by means of which the lights 

 are moved backwards or forwards, by an observer who 



