x ray and screen fixed in position relative to an interposed object it is evident that a straight line passing from the point of the to any point of the shadow cast on the screen must pass through the corresponding point of the object as in one plane in fig then as if the be moved into another position while the object remains stationary the same will hold true and another line be obtained which must pass through the same point of the object thus the of those two lines will determine the position of the point in question this applied to foreign bodies in the is the principle of the method first described by with the and sensitive plate set in definite known relation to each other two are made of the part the foreign body and between the two the is moved through a definite measured path parallel to the plate then the relative positions of and plate are and rays from the point of the in its two positions are represented by threads passing to two corresponding points of the shadows formed of the foreign body by the crossing of these threads is indicated the position in space of that point of the object relative to the plane of the plate during exposure to make possible this of the relative positions and to the ascertained position in space into terms which may direct a surgeon several points must be carefully attended to and several more or less complicated arrangements have been devised and two similar arrangements of a simple type for the special tables are also made some with a section and plate changing box below while the is set above the patient others allowing exposure with the below but the principle is the same in all on the plate in figures on the transparent part of the table or in the simplest arrangement on the plate are two cross wires at right angles and under these the plate must be placed so as to have their position on it in exposure of foreign bodies with the patient in position these wires leave on his skin f an must be made permanent either by previously the wires or by later tracing with an pencil the x ray must first be set with the above the crossing of the wires at a of about lo inches or the actual height may vary without affecting the result but when fixed the distance between the and the sensitive plate must be carefully measured and noted this of the may be assisted by use of a line or by the on the upright in i ig c which the latest form of the in that form the scale marked on the upright a convenient and method of measuring and the distance of the from the sensitive plate for this purpose the upright is at its base it is first set up on the plate or and the piece without the t in figure is set to the so that its two ends are in the same plane as the the height may then he read on the scale and noted but the upright should be removed without the position of the b the h k upright practical x ray work so that when its base is set on the as in the figure the position of the is directly the position of the foreign body having been observed by a previous the patient is now placed in position so that the object will lie somewhere near the centre of the area crossed by the wires those may be covered meanwhile by a board until the patient is placed and comfortably settled then the board removed place a coin or other mark such as an indicating number above the fig plate in one of the cross wires and mark the same on the patient s skin so as to prevent after confusion in the record the patient thus set move the x ray through a distance of as provided for on the arm of its this arm as in and must be set parallel to one of the cross wires so that the travel of the will be in the same plane the actual extent within limits of this movement like the height does not affect the final result provided that the distance be measured and noted so that it can be exactly later if the sensitive plate be not in position the may now be of foreign bodies and then the plate placed in position under the the plate being in position make a first then move the to a point other distance as noted on the other side of its central position and make a second exposure this second exposure may he made on the same plate as the first but is made on a fresh plate one plate may serve if the part exposed be very thin and the foreign body very dense or if a plate changing box be not provided and changing of the plate be therefore liable to disturb the setting of the part before removing the patient see that the cross wires are well marked on his skin along with an indication of the which is marked on the plate then develop the plates and when they are dry proceed with the next part of the this may be done with loss of time if drying of the plates be hastened after thorough by a bath of spirit or in pig the latest form of the cross thread now employed to interpret the recorded on the glass surface of the table are marked two a crossing at right angles which correspond to the cross i l on the i e practical x ray work wires on the plates exposed this surface is illuminated from below by a plane mirror the angle of which is the upright already referred to with the addition of a t 