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350 THE ADVANCE OF PHOTOGRAPHY
accessory which is found very useful in reducing or increasing the hardness of the tube, as may be required. It contains a suitable substance which has the power of lowering the resistance of the tube when a current is passed through it. Many methods have been tried by various experimenters for decreasing the hardness of a tube ; one method, that of heating the tube very gradually with a bunsen flame, until the glass becomes soft, being perhaps a favourite. Auxiliary side tubes containing platinum have also been used, as when such a tube is heated the contained platinum gives off a gas and so lowers the vacuum.
These accessory tubes are much easier to manipulate, much safer, and also more reliable, as, after a Rontgentube has been heated up a few times, its hardness can no longer be reduced by that means, and then the tube must be laid on one side for some months before it can be used again with the same sized coil.
If the vacuum is too high, the adjustable spark-point, at the top right of the figure, should be moved so that it is within about one-half inch of the bottom point, and the current then turned on for a few seconds. This causes a very small quantity of gas to be driven from the smaller to the main tube, and in this way the hardness of the tube is diminished. The distance between these two points may be arranged so that sparking takes place when the hardness reaches a certain degree, and then of course the tube will regulate itself automatically. Should the tube be very hard, and a great change be desired, then the wire from the kathode should be removed and connected to loop 1 on the auxiliary tube. The current should then be turned on and allowed to run for a few seconds, taking great care not to prolong this, as the tube will by so doing become too soft to be of use.
If it is desired to increase the hardness quicker than can be done by constant use, then the top point must be