Management of Light in Illumination. 185 



eter, and by taking the photometer in both his hands 

 he must turn it round till one of these shadows (that, 

 for instance, which belongs to the cylindrical column 

 on his left hand) comes into contact with the vertical 

 line which divides the field of the photometer into two 

 equal parts ; the whole of the shadow being on that 

 side of that line on which the column is placed, that is 

 to say, to the left of it, if it be the shadow of the left- 

 hand column, otherwise on the other side of it. 



As soon as one of the shadows shall have been thus 

 brought into its proper place by moving the photome- 

 ter about its axis, the other light must be moved by 

 an assistant to the right or to the left, till the second 

 shadow be likewise brought into its proper situation, or 

 till it comes into contact with the other shadow in the 

 middle of the field of the photometer. 



If the flames of the two candles happen to be at 

 the same horizontal level, the shadows which belong 

 to them will be at the same height in the field of the 

 photometer; and, if they happen to be at the same 

 elevation as the field of the photometer, these shadows 

 will just touch the horizontal line which is drawn 

 through the field of the photometer, at the level of the 

 upper extremities of the two columns. 



As this is the most favourable situation for the 

 shadows, they should always be made to occupy it; and 

 this may easily be done even without altering the ele- 

 vation either of the candles or of the photometer, by 

 means of the three wooden screws on the lower ends 

 of which the photometer rests. 



By elevating or depressing more or less one or both 

 of the hindermost screws, 2 and 3, Fig. 6, the extremi- 

 ties of the cylinders, the flames of the two candles, and 



