326 Messrs. Trowbridge and Sabine on 



between which the discharge took place. A concave silvered 

 glass mirror, of 313 centim. radius and 16*5 centim. aperture, 

 placed at a distance of 230 centim. from the spark, received 

 the image of the spark and reflected it back to the revolving 

 mirror. From the revolving mirror the image was reflected 

 to a photographic plate, which was at a distance of 259*7 

 centim. from the rotating mirror. 



The adoption of a plane revolving mirror, and a stationary 

 concave mirror of long focus, enabled us to place the photo- 

 graphic plate at a distance from the revolving apparatus, and 

 therefore to employ less speed for the revolving mirror. 

 There was no sensible aberration of the image. Great care 

 was taken to balance the mirror. Its large size and weight 

 made it very important, on account of the danger of the appa- 

 ratus flying apart, that it should revolve with uniformity. 

 The axis of the mirror was placed horizontally. This pre- 

 caution proved to be a wise one, for twice during the course 

 of the many runs which were made the mirror flew into 

 pieces ; the excursions of the fragments, however, were con- 

 fined to a vertical plane. This liability to accident is perhaps 

 inherent in a method which employs a large plane mirror. 

 The increased amount of light which results from the use of a 

 large mirror, however, forms a valuable compensation. The 

 revolving mirror was driven by a gas-engine. 



In order to determine the speed of the mirror at the instant 

 the spark passed, the following apparatus was devised. The 

 same shaft which carried the revolving mirror also carried a 

 brass cylinder 5 centim. in diameter and 21 centim. long. 

 This cylinder was covered at each trial with paper which was 

 coated with lampblack. A stylus moving along a stationary 

 rod beside the shaft could be made to draw a spiral upon the 

 revolving cylinder. One terminal of a Ruhmkorff-coil was 

 connected with the brass cylinder, and the other with the 

 stylus. A second pendulum was made to break the circuit of 

 the primary of the Ruhmkorff-coil at intervals of one second, 

 and at the middle point of its swing. When the stylus was 

 drawn along the stationary rod which served to guide it, it 

 was made to release automatically at the beginning of the 

 second another pendulum held up by an electromagnet. This 

 latter pendulum, at the middle of its swing, discharged the 

 air condenser through the inductance-circuit at the instant 

 that the mirror was in a suitable position to reflect the image 

 of the electric spark into the photographic camera. While 

 the stylus was being drawn upon the revolving cylinder, the 

 spark from the Ruhmkorff-coil left its trace upon the blackened 

 paper. The record on the chronograph consisted of a strongly 



