Research or some Standards of Light. 



473 



Fig. 1. 



To Photometer. 



full view, and good results might have been obtained in this manner. 

 Unfortunately this disposition had to be abandoned, as it soon became 

 evident that it was impossible for one observer to adjust the currents 

 in the three independent electrical circuits, to feed up the four carbons, 

 and to make all the electrical and photometric readings. 



The disposition which was finally adopted is shown in fig. 3. The 

 diaphragm d screws into a screen s. This screen is supported on a 

 ■system of pivots and levelling screws, so that it can be raised, lowered, 

 or turned round a vertical or horizontal axis. The opening d can 

 thus be directed to any portion of the crater. The positive carbon is 

 horizontal, and so placed that its axis coincides with the axis of the 

 photometer. 



Two distinct series of experiments were carried out : one with the 

 arc placed in a metallic enclosure kept at about 20° C. by a water 

 circulation, the other in the enclosure shown in fig. 3. The tempera- 

 ture of this enclosure varied from the melting point of silver to near 

 the melting point of platinum, according to the amount of power 

 expended in the arc. Referring to fig. 3, c is a carbon crucible sur- 

 rounded by a thick layer, b, of firebricks and refractory clay. The outer 

 covering a is of asbestos. Both the high and low temperature enclo- 

 sures were provided with a small camera obscura (not shown in the 



