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82 obtained with gigantic mirrors or prisms, but when we come to study the phenomena of polarisation as exhibited by crystals, Nature imposes a limit, and this limitation of the size of the crystals has to be accepted in conducting any investigation on their polarising properties.

The greatest drawback, however, in conducting experimental investigations on the optical properties of electric radiation arises from the difficulty of constructing a satisfactory receiver for detecting the radiation. For this purpose I at first used the original form of coherer made of metallic filings as devised by Professor Lodge. It is a very sensitive detector for electric radiation, but unfortunately its indications are often extremely capricious.

The conditions for a satisfactory receiver are the following:—


 * (1) Its indications should always be reliable.
 * (2) Its sensitiveness should remain fairly uniform during the course of the experiment.
 * (3) The sensibility should be capable of variation, to suit different experiments.
 * (4) The receiver should be of small size, and preferably linear, for accurate angular measurements.

These conditions seemed at first almost impossible to attain; the coherer sometimes would be so abnormally sensitive that it would react without any apparent cause. At other times, when acting in an admirable manner, the sensitiveness would suddenly disappear at the most tantalising moment. It was a dreary expeiience when the radiator and the receiver