322 Prof. J. A. Fleming and Mr. J. E. Petavel : 



observed on a graduated scale. It will be clear, then, that if 

 the disk driven by the motor was in synchronism with the 

 alternating current producing the alternating arc at A, an 

 observer, looking through holes in the rapidly revolving disk, 

 would see by reflexion in the mirrors M x and M 2 the alterna- 

 ting current arc at A ; but he would see it, not as it is seen 

 when looked at directly, but in some constant condition taken 

 at one definite instant during the phase, which instant would 

 depend upon the position of the line ~M X M 2 with regard to the 

 vertical line through the centre of the disk. Thus by rocking 

 over the arm MjMg i n various positions the observer coald 

 see, through the window W as the disk revolved, the arc at 

 A, either in the condition when the electric discharge is 

 taking place, or when the true arc is extinguished, according 

 to the position in which the arm M : M 2 was set. If, instead 

 of observing with the eye, a disk of paper with a photometric 

 grease spot upon it was placed at P, the lens L could be so ad- 

 justed as to throw an enlarged and well-defined image of the 

 arc upon the disk at P; and by rocking over the arm carrying 

 the mirror into successive positions, the observer woald see 

 the image of the arc pass slowly through all those successive 

 phases which in the arc itself actually take place during one 

 periodic time. As the image of the arc is much larger than 

 the photometer disk, it was possible, by slightly shifting one 

 of the mirrors, to bring any desired part of the image of the 

 true arc or of the craters of either of the carbons to cover the 

 grease spot. In addition to this interrupted ray, another ray 

 was gathered from the same part of the arc by a lens L 2 , 

 placed on the same level with the lens L 1 but slightly to one 

 side of it. This lens gathered a beam which was reflected by 

 a mirror M 3 placed at an angle of 45°, and which reflected 

 the ray upwards to another mirror M 4 . In fig. 2, for the 

 sake of clearness the lens L 2 is shown beneath the lens L 1} 

 but it must be understood that in the real apparatus the 

 lenses L x and L 2 w 7 ere on the same level and placed side by 

 side. The ray reflected from the mirrors M 3 and M 4 was set 

 horizontally, so as to be received on a lens L 3 , and by this lens 

 L 8 was gathered to a focus at a point I. The lens L 2 was so 

 adjusted as to form a large image of the arc on the screen 

 which carried the lens L 3 , and by slightly moving the mirror 

 M 4 any part of this image could be made to cover the lens L 3 . 

 It will be seen, therefore, that the light gathered together at 

 a focus at the point I could be made to be light coming 

 from any assigned area in the arc or from the craters. A 

 movable stand carried two other mirrors M 5 and M 6 fixed at 



