Patent Number: 
Section: claims

1. A filter which includes a stack of foils which are locally attached to one another including a pair of oppositely and substantially parallel disposed rigid members, forming top and bottom filter surfaces, between which pair of rigid members a stack of foils are disposed, wherein movement of the rigid members away from each other enables the foils to be moved away from one another in a rain direction which extends transversely to the surfaces, ducts being formed between the foils including walls coated with electrically conductive material in order to control an amount of x-ray absorbing liquid to be contained within the ducts, wherein at least one of the rigid members is attached to an outer surface of the stack of toils by way of a buffer member, wherein the buffer member is contractible mainly in a direction extending parallel to the surface of the foils and transversely to the ducts, and wherein the buffer member includes a number of laminations, each of which is rigidly connected, near a first edge, to a plate which constitutes the rigid member, each lamination being connected to one of the outer surfaces of the stack of foils by way of a second edge which is remote from the first edge, the second edge extending parallel to the ducts and being movable towards the first edge while the foils move away from one another in the main direction. 2. A filter as claimed in  claim 1 , wherein the laminations extend parallel to one another after the foils have moved away from one another over a given distance. claim 1 3. A filter as claimed in  claim 1 , wherein the buffer member is connected to one of the outer surfaces of the stack of foils by means of an elastic means. claim 1 4. A filter as claimed in  claim 3 , wherein the elastic material is a two-side adhesive layer. claim 3 5. An X-ray apparatus which includes a control device, an X-ray source, an X-ray detector, a filter as set forth in  claim 1  which is arranged between the X-ray source and the X-ray detector and includes ducts and an X-ray absorbing liquid which is contained in the ducts, the quantity of X-ray absorbing liquid variable in the individual ducts to vary the X-ray absorptivity of the ducts, which quantity and consequential absorptivity being adjustable by means of the control device. claim 1 6. A filter which includes a stack of foils which are locally attached to one another including a pair of oppositely and substantially parallel disposed rigid members, forming top and bottom filter surfaces, between which pair of rigid members a stack of foils are disposed, wherein movement of the rigid members away from each other enables the foils to be moved away from one another in a main direction which extends transversely to the surfaces, ducts being formed between the foils including walls coated with electrically conductive material in order to control an amount of x-ray absorbing liquid to be contained within the ducts, wherein at least one of the rigid members is attached to an outer surface of the stack of foils by way of a buffer member, wherein the buffer member is contractible mainly in a direction extending parallel to the surface of the foils and transversely to the ducts, and wherein the buffer member is provided with a spring which includes turns, the rigid member extending through the turns and the turns being connected to one of the outer surfaces of the stack at a side which is remote from the rigid member. 7. A filter as claimed in  claim 6 , wherein the rigid member includes two rods extending parallel to one another. claim 6 8. A filter as claimed in  claim 1 , wherein the rods are made of metal. claim 1 9. An X-ray apparatus which includes a control device, an X-ray source, an X-ray detector, a filter as set forth in  claim 6  which is arranged between the X-ray source and the X-ray detector and includes ducts and an X-ray absorbing liquid which is contained in the ducts, the quantity of X-ray absorbing liquid variable in the individual ducts to vary the X-ray absorptivity of the ducts, which quantity and consequential absorptivity being adjustable by means of the control device. claim 6