The invention is concerned with a novel system for making the measurements required to determine the relative position of the two ends of a tube, and the invention also relates to a novel process and to a novel apparatus for measuring the dimensions of the ends of a tube. The invention applies to a great variety of methods for connecting tubes to one another, such as methods which make use of brazing techniques, for instance, and the invention can advantageously be used in the case of a tube to be bent which is made by a mandrel that follows an accurate path, so that the tube bends about a bending line which is precalculated, on which the bends are made over at least one predetermined angle. The invention is also applicable to an apparatus for measuring dimensions of the end of a tube which is made by hot brazing. When tubes are connected together by brazing techniques, one known means of connecting tubes together comprises interposing a sleeve between the ends to be connected. The sleeve is used to enable the tube to be heated when making brazing, by having its exterior surface in close contact with the ends to be connected. When the brazing is complete, this sleeve is removed. For this purpose, it is essential that the relative positions of the ends of the tubes be known accurately so that there can be sufficient clearance between the ends to enable them to be connected together after the sleeve has been removed. The relative position of the two ends of a tube is determined using well known methods based on triangulation. In this method, the position of one end of the tube is known from optical means and the position of the other end is determined from the measurement of angles between optical axes at the points at which the triangulation lines intersect. To determine the position of a point on a tube with respect to a reference point on the other tube, for instance, three optical means may be placed on this tube at angles .alpha., .beta. and .gamma., and the relative position is determined by the angles at which the light rays intersect. An example of this technique is described in an article by Y. Saito and J. Fukuhara, "Measuring Distances by Triangulation", in "Introduction to Automated Engineering Systems", Proceedings of the 2nd International Conference on Automated Systems, pages 79 to 84, published in the United States by D. Van Nostrand Company, 1966, and in French patent specification No. 1 499 963. The known methods do, however, have the drawback that they require a great deal of space and that they cannot be readily applied to tubes to be connected by brazing techniques because of the space required for the placement of optical means. They are further limited in their applicability since they can only be used when the ends of the tubes to be connected together are parallel or nearly so. If the tubes have other dimensions, the system of angles defined by the two ends cannot be used. Moreover, if a tube has defects at its end, these methods cannot be used, since the end of the tube must be sufficiently flat to enable the light rays to intersect. For these reasons, no satisfactory method has yet been developed for determining the relative position of two ends of a tube in the particular case where they are not parallel or are otherwise different, where the tube is bent and where it is important to be able to know the relative position of its ends before removing the sleeve that follows it. The purpose of the present invention is therefore to provide a measuring apparatus and a process of measuring the relative position of the ends of tubes which can be used satisfactorily when the tubes are not parallel or are otherwise different and when they are fitted with sleeves, and which can be readily used in place of triangulation methods to determine the relative position of the ends of a tube, even if the tube has defects at its end. In order to satisfy this purpose, the present invention provides a measuring apparatus for determining the relative position of the ends of a tube by triangulation, comprising a support on which a reference triangle is made up of three marks each having an angle .alpha., this apparatus including means for measuring the dimensions of the tube with respect to the support. The present invention further provides a process of measuring the relative position of the ends of tubes by triangulation, comprising arranging the reference triangle of the measuring apparatus referred to above on one end of a first tube, placing a second tube on the end of the first tube which carries the triangle and carrying out a measurement of the angle .alpha., with respect to the axis of the first tube, of each mark of the first triangle which is located opposite a mark of the second triangle. By this means it is possible to use triangulation methods which make use of the relative position of the ends of the tubes. One of the advantages of the present invention is that it is independent of the shape of the ends of the tubes since the apparatus only needs to be capable of measuring the dimensions of one end of the tube, which need not be perfectly cylindrical. Thus it is not necessary to measure the diameter of the tube, which enables the reference triangle to be made smaller and the means for measuring and determining the dimensions to be made more accurate. The present invention makes use of the fact that if the ends of a tube which is not necessarily cylindrical are fixed with respect to one another, if all that is sought is to know the relative position of one end with respect to the other, it is possible to determine this position from the position of one of the ends with respect to a third reference position. The principle of the invention lies in the use of an additional piece of equipment, namely a reference triangle, with means for making measurements with respect to this reference triangle, for which purpose it is no longer necessary to know the position of the tube with respect to an outside reference position. The position of the reference triangle with respect to the reference position can be given by the operator, who can use his experience or the assistance of optical means to know the position of the ends of the tube relative to each other. By placing the reference triangle on the first tube, the relative position of this reference triangle can be determined with respect to the second reference position, for instance, which enables the relative position of the ends of the tube relative to each other to be determined by triangulation. It will be understood that when the present invention is applied to measuring the position of a tube with respect to the ends of a straight bar, two separate references are used and each reference triangle is assigned a single end of the straight bar. The two end points of each reference triangle can be determined from the measurement taken in one reference position. In order to provide greater accuracy in measuring the relative position of the ends of a tube with respect to each other, the present invention also provides an apparatus for measuring the relative position of the ends of a tube which includes three optical means placed in succession at angles .alpha., .beta. and .gamma. with respect to the axis of a tube to be measured, said means each comprising a light source and a sensor which can detect light rays reflected by the ends of the tube and which each provide a signal S.sub.1, S.sub.2 or S.sub.3 representing the dimensions of the tube. The apparatus includes means for calculating the relative position of the ends of the tube from the angles between the optical axes at which the light rays intersect and from the signals S.sub.1, S.sub.2 and S.sub.3 provided by the sensors. The calculation means preferably includes means for calculating the angle .phi. between