the eye is useless, the ear is invaluable. In working in this region of the spectrum, lampblack alone may be used in the spectrophonic receiver. Indeed, the sounds produced by this substance in the ultra-red are so well marked as to constitute our instrument a most reliable and convenient substitute for the thermopile. A few experiments that have been made may be interesting.
1. The interrupted beam was filtered through a saturated solution of alum.
Result: The range of audibility in the ultra-red was slightly reduced by the absorption of a narrow band of the rays of lowest refrangibility. The sounds in the visible part of the spectrum seemed to be unaffected.
2. A thin sheet of hard rubber was interposed in the path of the beam.
Result: Well-marked sounds in every part of the ultra-red. So sounds in the visible part of the spectrum, excepting the extreme half of the red.
These experiments reveal the cause of the curious fact alluded to in my paper read before the American Association last August—that sounds were heard from selenium when the beam was filtered through both hard rubber and alum at the same time. (See table of results in Fig. 14.)
3. A solution of ammonia-sulphate of copper was tried.