1 82 Management of Light in Illumination. 



tensities of the light immediately without any calcu- 

 lation. 



Fig. 6, Plate VIII., is a perspective view of this new 

 photometer. (See page 163.) 



« is a quadrangular wooden box, turned upside down 

 and fastened by means of wood screws or nails to the 

 board b. 



This board is 10 inches in length, 8 inches in width, 

 and I of an inch in thickness, and it rests on the lower 

 ends of three wooden screws, i, 2, and 3, by means of 

 which the board may either be placed in a horizontal 

 position, or inclined a little to the plane of the horizon, 

 as the occasion may require. The screw 2 cannot be 

 seen, being hid by the inverted wooden box. ^ is a 

 vertical board, which is fastened to the back side of the 

 box by means of screws, and which projects three inches 

 above the level of its inverted bottom. 



This board, which forms the field of the photometer, 

 is covered in front by fine white paper, and on this 

 paper are drawn with a pen two fine black lines cross- 

 ing each other at right angles. One of these lines is 

 vertical, and divides the field into two equal parts ; the 

 other, which is horizontal, is situated at the height of 

 two inches above the level of the upper surface of the 

 small table, which is formed by the bottom of the in- 

 verted box. 



On this table are drawn (with the point of a pair of 

 compasses) two straight lines at right angles to each 

 other, and in such a manner as to divide the table into 

 four equal quadrangular parts. 



This table is 7 inches in length and 5 inches in 

 width, and in the line which divides it in the direction 

 of its length are placed two vertical pillars or small 



