vacuum. These are called adjustable vacuum tubes, and afford a means of controlling the requisite sparking gap of the coil within certain limits. Nos. (4) and (5) are now almost the only styles of tubes that meet with favor.
Three types of apparatus have been employed in exciting the X rays. All are necessarily such as are capable of producing a high electric potential, and all were in use prior to Dr. Röntgen's discovery. They are the Ruhmkorff induction coil, the plate influence machine (either the Wimshurst or the Töpler-Holtz), and the Tesla coil. The only development in these machines has been in some instances the improvement of their quality and enlargement of their capacity—without, however, introducing any novelty in the type of the apparatus, unless we except making the condenser of the induction coil adjustable in capacity. The most suitable rate of interruption of the primary current for each coil and tube may best be found by trial. Where a continuous current is supplied from a commercial circuit of a hundred and ten volts or more a rotating segmental wheel as interrupter with a rheostat in circuit is of advantage, but many experimenters get as good results by using a storage battery of six to ten cells, with an ordinary hammer break in the coil. The Ruhmkorff coil is used to give a unidirectional discharge in the Crookes's tube. Influence machines having several rotating plates act in the same way, and with excellent effect. Tesla coils are employed to give exceedingly rapid discharges to and fro in the tube, which require, therefore, two terminals that can both act as cathodes. It can not be said that either of these three forms is per se the best. With proper accessories one will give as good results as another, but the ordinary induction coil with suitable single-focus tube is the most generally practicable.
Fig. 4 shows an outfit of apparatus for X-ray use. It consists, in this instance, of a variable rheostat connected to one main of a hundred-and-ten-volt continuous current; in series with this is, next, a rotary interrupter, which is also driven by a current from the same main circuit; then comes an ammeter, then a pole changer or reverser, which connects back to the other main and forward to the primary of the large Ruhmkorff coil. This coil has in its base a condenser which is united with an additional adjustable condenser. There are, further, a double-focus tube, fluoroscope, and screen.
The most obvious suggestion of usefulness for the new agent was in surgery. It was so easy to discover any foreign substance in portions of the body, or to perceive the nature of any bony malformation, that it was hoped that surgery had received a valuable assistant in these rays. From time to time reports of successful operations based upon such revelations have been made,
