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The extreme sensitiveness of the quadrant method is demonstrated by the record given in figure 94.

The duration of a spark-discharge of a Leyden Jar may be regarded as of the order of hundred thousandth part of a second. The discharge took place at a distance of 15 cm. from the leaf and the response is seen to consist of a preliminary positive twitch followed by a large negative response, indicative of normal diminution of resistance. The leaf exhibited a complete recovery.

The arc lamp is taken out of the lantern, and the diverging beam employed for the following experiment. As the intensity of light varies as the square of the dis-