312 



raised 1 cm. from A by a block of wood under each of the four corners 

 as at C to allow the to-and-fro motion of the pointer D. The longitudinal 

 opening under G and the base of D give ventilation. The figures on the 

 side of B are accurately placed on the original apparatus and show the 

 intensity of light in Hefner meter units. At E^ is a mirror and at F a 

 circular opening to a mirror inside. The interior view is shown in Fig. 

 8. Here A is the wooden base, C the wooden block supporting the box 

 B. L and L' are strings by means of which the Hefner-Altneck amyl- 

 acetate lamp is regulated in distances from the membrane K. This mem- 

 brane consists simply of a circular piece of filter paper covering an open- 

 ing in the center of the end of the box 7.5 cm. in diameter. In the center 

 of the filter paper is a circular spot about three cm. in diameter, as in the 

 ordinary Bunsen photometer. Each mirror E E^ is 5 cm. long and 4 cm. 

 wide and is inclined at an angle of about 45° to the filter paper membrane 



z 



r-E: 



>^ A 



Fitr. s. 



by means of the galvanized iron frame which supports it. The V-shaped 

 lop G is also composed of galvanized iron. This apparatus will show the 

 intensity of light from 2 to 90 Hefner meter units. When it is desired 

 to test the intensity of light between two and ninety Hefner units, draw 

 the pointer D, opposite which inside the box the Hefner-Altneck amyl- 

 acetate lamp is placed, by the string L towards the filter paper mem- 

 brane. When the circular spot of paraffin-^ on the filter paper almost dis- 

 appears, then the light cast on the filter paper screen by the amyl-acetate 

 lamp, and that in the room for example otitside are equal. Noticing now 

 the position of the pointer D. we notice that it points to or near some 

 figure on the outside of the box. The number it points to indicates the 

 intensity of light in Hefner meter tmits. 



