Methods of Altering

1  Nature of Tubes.

I. Softening may be necessary to utilize an old tube, or to adapt any tube for a special purpose requiring a lower degree of penetration. This may be effected in various ways :

(a Use of regulators, as described above, is the preferable method.)

In the absence of a regulator—

(6) Baking the tube for Borne hours in an oven at a high temperature may somewhat soften a tube per- manently.

(c) Laying the tube aside for a year or more induces some degree of softening.

(d) Heating the tube carefully, by the flame of a spirit-lamp or before a fire, softens it temporarily, but on cooling the previous hardness is reassumed.

A tube may be thus softened while in the live circuit (that is, with current passing, or tending to pass, through it). Thus natural softening may be hastened, or the vacuum initially reduced, to allow discharge through the tube to commence. In such a case the spirit-lamp employed should be held on a long insu- lating handle, so as to protect the operator from shock or X-ray burn. Care must be taken in so heating a tube that no part is over-heated, as a dent may be readily produced in the softened glass by the atmospheric pressure outside acting against the much-reduced pressure within.

All softening processes should be carefully regulated, and must not be carried too far, since hardening is difficult, and always more or less injurious to the tube.

II. Hardening should never be called for where a graded stock of tubes is kept as advised, and where due care is exercised in use of the tubes or in any softening process called for. Where under special conditions hardening may be necessary, it is best done by—

(a) Sending the current through the tube in the inverse direction for a short time, the effect being similar to that described as causing the gradual harden- ing of tubes through continual use.

(b) Villard’s method, by heating an open tube or sleeve placed over an osmo-regulator, whereby the ordinary softening function of that arrangement is said to be reversed and hydrogen drawn out of the tube, is un- satisfactory in practical use. This method is illustrated in Fig. 9.

(c) The automatic side-tube regulator, represented in Fig. 7, is also said to be available for the purpose of hardening. To do so, it is advised to remove the positive wire from the anode of the X-ray tube, and to connect it to the terminal at that end of the side-tube, the negative wire being connected to the kathode ter- minal as in ordinary excitation of the X-ray tube. The side-wire of the regulator must be well separated from the kathode terminal, and current then passed through. We have not used this method frequently, nor do we recommend it.

On the rare occasions on which hardening has been called for, we have found the first-mentioned method the most serviceable.

IV. Means to prevent over-heating- of the antikathode are various:

(a) Heavy metal antikathodes, as in Gundelach’s tube, illustrated here, serve well, the increased mass of metal taking much longer to become injuriously hot than the ordinary thin discs employed. For the first twenty or thirty excitations such tubes are disappointing, since they soften rapidly, and to a marked degree. This is due evidently to the occluded gases unavoidably present in the metal, but after a time these become exhausted. Then the tube works steadily, and will stand long runs with heavy currents very satisfactorily.

We would suggest that a new tube of this type be used for therapeutic purposes for a time, its condition being carefully noted meanwhile before reliance is placed on it for radiographic work.

(b) Water-cooled tubes have the antikathode and stem surrounded by a water-jacket, which retards the heating of the target. Special attention must be paid to the position of the tube, so as to have the water in the jacket always in contact with the target. Thus the range of use of such tubes is limited. Fig. 11 shows in diagram a design allowing adjustment of plugs to suit the position of the tube.

(c) Air-cooled tubes of similar principle are a recent modification, and, in our experience, seem to be a very efficient arrangement for the purpose, being very con- stant in action and capable of being used in any position. Their price is so far somewhat prohibitive for ordinary use, and more experience of their working is desirable before finally judging them. A good example of this tube is the ‘ Tantalum/ as shewn in Fig. 12A.

V. The metal parts have been modified in various ways, as mentioned earlier, to avoid disintegration and damage from over-heating. Since the main cost of a tube is in the labour of manufacture—and this is the same for whatever material be employed—it is obvious that the best suitable material should be insisted upon in all tubes, however their design or construction may be modified to lessen their price. The purity of metals employed is of prime importance, especially in the case of the kathode and its supporting stem, for which aluminium has proved to be pre-eminently suitable.

Much of the value of a tube depends upon the construc- tion of the antikathode, since the quality of the resultant radiation seems to depend largely upon the * stopping power ‘ of the material opposed to the kathode rays. Probably in no case are the kathode rays all stopped by their first impact on the surface molecules of the target, but some rays penetrate to deeper molecules before being stopped and giving origin to the resultant X rays. Thus radiations are produced, as it were, from successive rows of molecules ; and this would seem to explain the origin from one tube of a collection of rays of differing qualities. Those rays produced by the stoppage of kathode rays at the first row of molecules are of the highest degree of penetration, and probably also actinic effect. Platinum, of all the metals tried, possesses the highest stopping power, and the heavy anodes of pure platinum employed in some of the most expensive tubes give beautiful effects. But pure platinum is very liable to over- heat, and probably the best all-round antikathodes are now made of platinum alloyed with a proportion of pure nickel, thus combining stopping power with resistance to over- heating.

Modifications of X-Ray Tubes.

I. Addition of a third electrode is one of the earliest and most common modifications of the tube from the form designed by Jackson. This serves as an anode, placed axially opposite to the kathode, and is connected outside by a wire to the target antikathode, the stem of which passes through the tube wall in a line at right angles to the desired plane of the target.

This arrangement is said to steady the action of the tube in a manner variously explained, and also to prolong its working life by retarding the change in quality described above, but its service is questionable. In France and America it seems to be little used, though in the German pattern of tube, which is mainly followed in this country, it is almost universal.

II. A larger diameter is given to tubes than formerly, changes temporary and permanent being thus delayed, and the life of the tube prolonged. Originally tubes of 2£ inches in diameter were commonly used; now 5 or 6 inches is a common diameter; whilst tubes are made of 9 or 10 inches diameter, and may become more general.

Before choosing a tube of large diameter, however, one must ascertain whether the internal electrodes of the tube are set correspondingly farther apart, for this distance may be limited by the power of the coil employed. With a small coil say a 10-inch size—a tube with electrodes set far apart might soon become too resistant for the full power of the coil to excite, while a tube of smaller proportions under similar circumstances would permit excitation and emit rays of sufficient penetration for all purposes.

With tubes of proportions as ordinarily made we would suggest that—

With a 10-inch coil, a tube be used of 13 centimetres or 5 inches in diameter;

With a 15-inch coil, a tube be used of 15 centimetres or 6 inches in diameter.

With static machines smaller tubes are more serviceable, one of 8 centimetres or 3 inches diameter giving useful penetration.

Of coarse, makers could construct larger bulbs while main- taining the shorter distance between the electrodes; but the two dimensions usually vary in proportion,.hence the above caution should be remembered. …… .

III. A vacuum regulator of some kind is added to almost all modern tubes, except the very low-priced ones. Such an addition is an ultimate economy, since it counteracts the hardening effect of continued use, and thereby prolongs the period of usefulness of the tube.

These regulators, when brought into action, give off, or transmit, gaseous substance, whereby the number of elec- trons in the interior of the X-ray tube is increased, and the degree of vacuum correspondingly reduced. They are set in action by heat, produced either by electric discharge or by direct application of a flame to the regulator.

(a) A small side-tube containing a chemical (such as KHO), which gives off vapour when heated, may be attached in construction to the X-ray tube. When desired, this is heated by a flame applied to the outside, and gas is thereby driven off, which passes into the main tube. This early form is now superseded by more con- venient arrangements, but illustrates the elementary principle of such regulators.

(b) A side-tube with some capillary substance, such as woven glass, mica discs, asbestos, or spongy metal, may be similarly connected to the main tube, as shewn in Fig. 7. The capillary substance is arranged at one end of the side-tube, and is traversed or surrounded by a platinum electrode, to which, when put in circuit, sparks may discharge from an electrode in the form of an aluminium disc at the kathode end of the tube. By such discharge heat is generated, and the gaseous contents of the capillary substance are thereby caused to expand, and part to be expelled into the vacuum of the larger tube. . .

By force of magnetic attraction it draws the hammer (J)  towards it, and thus separates the two platinum points at (H).  This separation interrupts the flow of the current, the core  consequently loses its magnetism, and the hammer (J) returns  by force of its spring (D) to its former position. Contact at  (H) is thus restored, current passes as before in the primary,  the core becomes again magnetised, and this cycle of events is  repeated automatically as often as the succession of’ make ‘  and ‘ break’ at (H) will allow. The alternative path (dd) to  the condenser (ee) will be dealt with later in reference to the  suddenness of the break.

The secondary winding, which consists of many thousands  of yards of very fine insulated copper wire, is not shewn in  Fig. 41, but its position is within the bobbin (KK), and its  endings are seen emerging at (FF), from which points con-  nection is made to the X-ray tube.

Following the laws of induction, at each make of the  primary current a momentary current is set up or induced  in the secondary circuit in a direction opposite to that in  the primary ; and at each break of the primary a momentary  current is induced in the secondary in the same direction  as the primary.

Changes in X-Ray Tubes by Use.

I, By repeated use an X-ray tube becomes progressively harder. This is due to (a) inverse currents, and (6) escape, or occlusion of gaseous particles from the interior of the tube.

(a) The formation and prevention of inverse currents are’discussed later, when induction-coils fall to be con- sidered. Meanwhile, we note that inverse currents may originate in the secondary winding of the induction-coil, and pass through the tube as discharge in a reversed direction. In their passage these currents tear from the platinum antikathode fine particles, which absorb or occlude electrons from the rarefied contents of the tube and thus increase the degree of vacuum. Presence of such inverse currents may be noted in the action of the tube, since they produce a flickering of the fluorescence, specially noticeable in the hemisphere normally dark.

Due to this action, the fluorescent hemisphere of a tube, subjected to such adverse influence, becomes blackened by deposit of finely disintegrated metallic particles, in contrast with the violet tint due to chemical change acquired by a tube guarded from inverse currents. To obviate disintegration during correct operation of a tube, the kathode is generally made of aluminium, which is found to resist such action more than any other metal tried.

(b) Escape of electrons may occur by piercing the glass of a tube, impulse from within being so much greater than any pressure from without. The degree of vacuum in the tube is thereby directly raised.

II. During each operation there tends to be (a) a pro- gressive softening, or, under exceptional circumstances, there may be (b) a slight hardening.

(a) A progressive softening is noted when a tube is so operated that the antikathode becomes overheated by the continual bombardment of the kathode rays. This heating has the effect of liberating electrons otherwise held bound by the metal, and by the liberation of these into the space of the tube the degree of vacuum is lowered.

If the tube be over-driven, this effect may become so marked as to reduce the equivalent spark-gap to nil. In operating a tube, its condition should be observed periodically by approaching the discharging points of the coil or noting the reading of the milliamperemeter. If softening of the tube be indicated by a marked shortening of the alternative spark-gap, or rise in the reading of the milliamperemeter, care must be observed that the tube does not receive injury, or the patient be exposed to risk of over-effect. In such event it will be well to decrease the amount of current employed or to give the tube time to cool.

In use the antikathode should never be allowed to get hotter than indicated by a cherry-red colour, unless softening of the tube is desired for special photographic effects, as will be described later.

If the kathode rays were focussed to an actual point on the antikathode the metal would readily become fused by the heat. Therefore in practice the target, made of platinum, is placed a little to one side axially of the focal point, and the X rays originate from a small circular area measuring between TV and J inch in diameter. To permit of a nearer approach to the true focus, combined with prolonged use, it has been suggested to make the target of osmium or iridium on account of their greater hardness and infusibility, but the expense and trouble in working of those metals are incommensurate with the advantage to be gained by their use. This point is further discussed later (p. 20).

The focal area on the target is usually indicated by a slight roughening of the metal, the test-running by the maker being sufficient to produce this effect, and it should be looked to in selecting a tube. If larger than £ inch in diameter, there will be lack of definition in shadows cast by the tube.

(b) Slight hardening of the tube may occur, or softening process be delayed, by the presence near the tube of an insulated mass of metal such as in a diaphragm. Such metal acts as a condenser, and, holding electrons bound to the adjoining inner surface of the tube, reduces the number free to occupy the tube space, and thereby raises the degree of vacuum. This effect may be obviated by connecting to earth either the kathodal wire or the metal mass in question.

Quantity of X Rays.

The quantity of rays produced must be taken into con- sideration as well as the quality, and depends on a number of factors more or less inconstant. These factors jointly determine the amount of electricity passing through the tube, and in direct consequence determine the amount of X-radia- tion produced. They include voltage and amperage of current . supplied, periodicity and regulation of interruption, action of induction-coil, and nature of the X-ray tube. Most of these factors are under direct control of the operator, but in practice uniformity is difficult to maintain, and even with careful regulation variations occur. Also, as will be con-

sidered later, the X-ray tube may alter considerably in nature during a single operation.

Thus it is very difficult, from any or all of those initial factors, to estimate quantity of X-radiation, and time standards of exposure based upon them as data are unreliable. For long exposures for therapeutic purposes they may, indeed, be dangerous.

Only by noting the actual current passing at any moment through a tube in action, or by noting the actual effect, chemical or otherwise, of the.rays produced, can one judge the quantity of X rays, and calculate their probable effect. None of our present methods of measuring the quantity of radiation or its effect are wholly satisfactory.

By watching the register of a milliamperemeter inserted in the tube circuit, the production of rays can for ordinary purposes be best measured, and the time of exposure for any desired effect judged. If this record tends to vary during the operation a time average may be taken, or by regulation of the current supplied to the induction-coil the amount passing through the tube may be kept constant. Since from a soft tube a relatively greater quantity of rays is produced, it is of importance to observe any softening of the tube during exposure. This is of special importance in therapeutic work, since therapeutic action seems to depend for its intensity directly on the quantity of radiation in the exposure. This change, corresponding to a fall of electric resistance in the tube, will be indicated by a rise in the reading of the milli- amperemeter, and, conversely, hardening of the tube will be indicated by a fall in the reading.

Where a maximum therapeutic effect is desired at one exposure, as for epilation in treatment of ringworm, it is pre-eminently important that the degree of exposure should be carefully measured, for the margin of safety between epilation and a serious dermatitis is narrow. For this purpose Sabouraud’s pastilles are very serviceable, though by no means deserving to be considered as a final standard. These are exposed to the rays during the actual exposure, and by chemical change directly measure the quantity of radiation and indicate the probable therapeutic effect. Con

fiisting of platino-cyanide of barium, thickly coated on small discs of paper, the pastilles on exposure to X rays alter in colour from a canary-yellow to a brown. The change is pro- portional to the cumulative effect of the exposure. Thus, by comparison with a standard tint the operator can determine from the pastille when the desired degree of exposure has been reached. The conditions to be observed in using these pastilles will be found described in the section dealing with therapeutics.

At the meeting of the British Medical Association at Exeter a meter was described which registered the electrolytic effect of the current passing in the tube circuit. The effect is, of course, cumulative, so that the amount of gas liberated and measured in a audiometer tube will bear a direct ratio to the current which has effected its disassociation. This is a very ingenious device, and well merits attention; but, for maxi- mum exposures, we feel that we should like to understand more fully the vagaries of X-ray tubes before we depend on any other measurement than that directly made of the actinium effect of the radiation in question.

Chapter 1 – Part 2

Observation of Nature of Tubes and Quality of Rays.

The quality of the X rays produced is shewn above to depend upon the nature of the tube from which they are produced; hence it is very important to observe the nature of a tube during operation.

1. The colour in the tube, both of the fluorescent hemi- sphere in front of the antikathode and of the hemisphere behind that plane, varies according to the hardness; but this factor is too indefinite to give practical indication of other than gross differences. In a tube acting properly the hemi- sphere in front of the antikathode should show a bright apple-green fluorescence, while the hemisphere behind should be free from luminosity. In a soft tube the fluorescence is intensely green and uniform, while the gas in the tube shews a faint bluish luminosity plainly seen behind the antikathode. In a hard tube the fluorescence is thin and grey-green in tint, while irregular, flickering green spots are seen on the walls of the tube.

The penetration or penetrative power, dependent directly on the hardness, may be measured by several methods—

(a) By observing the shadow cast on a fluorescent screen by a hand interposed between that and the X-ray tube, the shadow cast by the bones approximating in density to that cast by the rest of the hand in direct ratio to the hardness of the tube. This is at the best but a rough, relative test, and is not to be recommended, as such repeated exposure of the operator’s hand may lead to a serious dermatitis.

(b) By radiometer or radio-chromometer, an instru- ment made on the same principle as the tintometer apparatus for estimating the haemoglobin of the blood. In this the power of the rays to penetrate a metal of uniform density but varying thickness is observed and compared with a standard. The instrument of Benoist consists of a thin central disc of silver surrounded by a flat ring of aluminium graded by steps in thickness from 1 to 12 millimetres. For use, hold this up so as to intercept the rays from a tube, observe the shadow cast on a fluorescent screen, and note the sector or step by which is produced a shadow of density similar to that of the standard disc in the centre. The sectors being numbered according to their thickness, the higher numbers will correspond to harder tubes. Modifications of this intrument have been introduced, some of them of value, but all are similar in principle.

(c) On the equivalent spark-gap more dependence in practical work is placed than on all other methods of indicating or estimating hardness or softness of tubes.
“When an X-ray tube is connected to an induction coil in the ordinary way, the discharge of the induced current may take place along two alternative paths, as is shewn dia- grammatically in the annexed figure (Fig. 5). Discharge by way of the X-ray tube produces the special phenomena in the tube; discharge by the other path across the gap between the two discharging points of the coil takes the form of a series of sparks, The current in discharging will always take the path of least resistance. The discharge-gap is variable at will by moving the points nearer or farther apart, and the resistance offered to discharge across these points will vary directly with the air-distance between them.

While a tube is in operation, if the points, at first far apart, be gradually approximated, a position will be reached at which discharge takes place through the air between the points in preference to passing through the tube. Conversely, if discharge is taking place across the points, and they are gradually drawn apart, then, after passing the above position, discharge will cease across the points, and will take place through the X-ray tube. This distance between the points, called the ‘ equivalent spark-gap/ or * alternative path,’ denotes the resistance and consequent nature of the tube.

In adjusting the distance between the discharge points care must be observed that the points are not allowed to come into direct contact, so as to make a closed circuit, for under such circumstances the induction-coil might be badly damaged by the heavy discharge of current permitted.

The arrangement is sometimes called a spintermeter, or spark-measurer, and the hardness of a tube is denoted by a number corresponding to the distance noted, which distance is indicated on the sliding-rod of the discharge-gap. Thus, tubes with an equivalent spark of—

1 to 2 inches are soft.
3 to 4 inches are medium.
Above 5 inches are hard.

For purposes of comparison, it is necessary that the electrodes or discharging points of the spark-gap should be uniform in form and dimensions, since the same current will discharge across the electrodes at a greater distance apart if their opposing points be sharp than if they be rounded.

More uniformity in this is observed on the Continent, but all workers in this country should also adopt the standard of two spherical endings of 1 centimetre in diameter.

To prevent perforation, when using a hard tube it is well to leave the spark-gap only a little wider than the working distance of the tube. Otherwise, if the tube becomes too hard, there is no alternative path provided, and the current, if ‘ pushed/ will pass between the electrodes of the tube through the air outside, or seek a shorter path from the kathode to the outside by piercing the glass. This ‘ perforation ‘ will destroy the vacuum, thereby rendering the tube useless, and repair is very difficult. If perforation occur, the tube will be seen to change rapidly and violet light to appear, while production of X rays entirely ceases.

Chapter 1 – Part 1

THE X-RAY TUBE

Classification of Tubes.

TYPES and modifications of X-ray tubes are many and varied; but variations are of secondary import, and all conform to the general plan designed by Jackson.

As mentioned in the introductory remarks on the evolution of the X-ray tube, much depends on the degree of vacuum existing in the tube under observation. This vacuum, attained initially by means of a mercury pump, and completed usually by passage of electricity, can be adjusted within certain limits by the maker at the time of manufacture, and a tube may thus be made to suit any specified set of conditions.

A tube in which the exhaustion has not been carried very far is spoken of as of low vacuum, or ‘ soft’; whilst a tube more thoroughly exhausted is by contrast of high vacuum, or penetrative.

A soft tube, as compared with a harder one—
(1) Permits passage of an electric current with a lower electro-motive force (E.M.F.) in the tube circuit;
(2) Produces grreater quantity of X rays;
(8) Emits rays which possess higher actinic power;
(4) Emits rays of lower penetration.

By penetration is meant the relative power of the rays produced to pass through objects interposed in their path.

This, rays from a soft tube will with difficulty penetrate bone; and if exposed to such rays the larger bones of the body will cast very deep shadows. On the other hand, a soft tube will reveal detail of structure in the smaller bones which would be entirely undefined by the more penetrating raya from a hard tube suitable for viewing denser structures.

Thus the degree of vacuum, with its concomitant power of penetration, will indicate the value of a tube for any specified purpose, and results will depend largely on the judicious choice of a tube. It is well, therefore, to have in use several tubes of varying degrees of hardness, and to let each be reserved for uses suitable to its special powers.

The present X-ray tube has been evolved from the older Oeissler or vacuum tube. In the latter, when the pressure is reduced slightly, the resistance to electric discharge is lessened, and discharge takes place through the tube with striking phenomena of illumination, depending in character on the degree of exhaustion. The chemical nature of the residual gas also affects the character of the illumination; but we are concerned only with the presence of ordinary air. Exhaustion beyond a certain degree increases the resistance to electric discharge through the tube; and as the resistance increases, the luminosity of discharge disappears. But at a point of high exhaustion a fluorescence becomes evident on the walls of the tube, or on any object interposed between the electrodes. In vacuum tubes made of soda glass this fluorescence is of a transparent apple-green colour; with lead glass it is of a bluish tinge.

Sir William Crookes, about 1891, studied and interpreted the phenomena of such discharge in high vacuum tubes, the pressure in the tubes he worked with being reduced to about
one millionth part of an atmosphere. He shewed the fluorescence to be due to a bombardment of the interposed object by streams of negatively charged particles, moving from the kathode of the tube at a very high velocity. Such particles are now spoken of as electrons. In tubes such as were experimented on by Crookes these electrons are considered to move with a velocity equal to about one-tenth the velocity of light.

Lenard, workmg later with highly exhausted Crookes tubes, named this stream of electrons ‘ kathode rays,’ and found that those rays also existed outside the tube. He shewed that they could pass through some substances opaque to ordinary light, could excite fluorescence on suitable substances, and could act on sensitive photographic plates.

Roentgen, one year later—about the end of 1895—discovered that along with those kathode rays proper there were emitted by such high-vacuum tubes other rays having some differentiating properties. These he named X rays.

One essential physical difference consists in the failure of a magnet to deflect these X rays, whilst kathode rays are so deflected; nor can X rays be either reflected or refracted. Thus, in contrast to the kathode rays, which consist of what may be termed a corpuscular stream, X rays are true ethereal rays.

These X rays are produced by the impingement of the rapidly moving kathode rays on any obstacle in their path. In the earliest 4ubes, as represented in Fig. 1, they were produced on the glass wall of the tube wherever it was struck by the rays proceeding from the kathode. That was in the form of a flat disc, whilst the anode was annular in form.

Herbert Jackson made important and essential alterations in design, and on his plan are constructed all X-ray tubes of the present day. The kathode was by him made of concave form, so as to focus the rays proceeding from it to the centre of the tube, and near that focal point was introduced a metallic target called the anode or antikathode. This target is set at an angle of 45 degrees to the axis of the tube, so as to throw the main part of the X rays to one side of the tube. This device rendered the study and employment of X-ray effects much more precise, since the rays proceed from a definite point or small area of the target.

Of the physical properties of X rays little need be said. The property upon which depends their use in medicine as a diagnostic aid is that of penetrating many substances opaque to ordinary light. The degree of penetration varies inversely with the density of the opposing substance. Thus, differentiated shadows are cast of the different tissues, and departures from the normal may be detected. Bone, being denser than muscular or other soft tissue, offers greater resistance to the rays; hence it casts a deeper shadow, and alterations in its density, as in necrosis, will influence the shadow cast, while such lesions as fracture will do so more markedly.

But to the naked eye X rays are not visible, and such interference with them not discernible ; hence two other properties are brought into requisition—that of rendering fluorescent certain substances such as the platino-cyanides of barium and potassium, and that of acting upon sensitised photographic plates.

The intensity and differentiation of the shadow cast on a fluorescent screen by a certain radiation will depend upon the nature of the body or substance interposed between the screen and the source of the transilluminating rays; and so likewise will depend the image impressed on a photographic plate exposed for a suitable time.

The effect of X rays on living tissue exposed to them is  discussed in the section on ‘ Therapeutics.’

A MANUAL OF PRACTICAL X-RAY WORK by DAVID ARTHUR, M.D., D.P.H.

INTRODUCTORY

To one already acquainted with radiology it matters little in what sequence we consider its various factors or problems.  He will seek out for himself the parts in which he may be  specially interested, or the points on which he may desire enlightenment.

As a general plan of the work, we have thought lit to begin by considering the immediate production of X rays; next to consider the remoter means of their production; then to proceed to the discussion of their practical uses. For the benefit of those who have not previously studied the subject, we have sketched the following brief introductory note on the evolution of the X-ray tube. Like so many other processes utilised by the medical profession, radiology is based on the results of painstaking research in pure science—research initially remote from any suggestion of therapeutic issue.

The present X-ray tube has been evolved from the older Oeissler or vacuum tube. In the latter, when the pressure is reduced slightly, the resistance to electric discharge is lessened, and discharge takes place through the tube with striking phenomena of illumination, depending in character on the degree of exhaustion. The chemical nature of the residual gas also affects the character of the illumination; but we are concerned only with the presence of ordinary air.