Page:Collected Physical Papers.djvu/381

Rh ; when the person walks past the line of sight the indicating spot of light returns to its original zero.

The zinc wire was surrounded by a wooden tube with a narrow slit for the passage of the spectral rays. The slit was open through a length of 10 cm., the brass piece to which the strip was soldered being protected from radiation. A very thin piece of mica covered the slit, to prevent air-currents getting access to the narrow chamber in which the sensitive strip was enclosed. The exceedingly thin piece of mica was found practically to have no effect in obstructing radiation. The whole apparatus was placed inside a larger wooden box covered with non-conducting felt. A round hole in front of the box (covered with a piece of thin glass) allowed the passage of the spot of light reflected from the mirror attached to the suspended magnetic needle.

It is necessary to give a detailed account of precautions to be observed, because the instrument is so sensitive that it detects the slightest difference in the temperature of the different portions of the same mass of air. Gases are highly non-conducting, and their temperature hardly attains a perfect uniformity. In these circumstances it is best to take precautions against contact of the strip with the outside air, which, again, should not be disturbed in any way. The observer never moves from his place in the dark room, which is closed on all sides.

The spectrum produced by the carbon disulphide prism was found to extend beyond the limit of the visible red, this extension into the infra-red region being almost 6 cm., or about one-third the breadth of the visible spectrum.