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A replaceable applanation prism for an eye examining instrument is made to be discarded after each use. A bendable tab molded on the prism is deformed by a strain gauge as the prism is inserted into a prism holder, and a signal from the strain gauge is used to verify that a previously unused prism h

A replaceable applanation prism for an eye examining instrument is made to be discarded after each use. A bendable tab molded on the prism is deformed by a strain gauge as the prism is inserted into a prism holder, and a signal from the strain gauge is used to verify that a previously unused prism has been inserted into the holder. A microprocessor is programmed to recognize the signal produced by initial deformation of the prism tab so as to proceed with an eye examination only after receiving the strain gauge signal verifying that a previously unused prism has been inserted.

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We claim: 1. A system ensuring replacement of a molded resin prism used in an applanation tonometer having a microprocessor and a holder for receiving the prism in an operating position, the system comprising: a. the prism being molded to form a bendable tab having an initial position extending tra

We claim: 1. A system ensuring replacement of a molded resin prism used in an applanation tonometer having a microprocessor and a holder for receiving the prism in an operating position, the system comprising: a. the prism being molded to form a bendable tab having an initial position extending transversely of a direction of insertion of the prism into the holder; b. a strain gauge arranged in the holder to engage the tab as the prism is being inserted into the holder and before the prism reaches the operating position; c. the strain gauge being arranged to bend the tab from its initial position to a bent position as the prism is moved fully into the holder to the operating position; d. the strain gauge being arranged to produce a tab bending signal delivered to the microprocessor representing the strain involved in bending the tab from its initial position to its bent position; e. the tab being configured so that a first bending of the tab from its initial position to its bent position requires more stress than any subsequent bending of the tab from its initial position to its bent position; f. the microprocessor being programmed to distinguish between a tab bending signal from the strain gauge representing the first bending of the tab and a tab bending signal from the strain gauge representing the subsequent bending of the tab; and g. the microprocessor being programmed to proceed with an eye examination only if a prism inserted into the holder causes the strain gauge to produce a tab bending signal representing the first bending of the tab. 2. The system of claim 1 wherein the prism has a location projection that detents in the holder when the prism is in the operating position. 3. The system of claim 1 wherein an optical axis region of an applanation surface of the prism has a small indent. 4. The system of claim 1 wherein a rear face of the prism has a reticule on an optical axis of the prism. 5. An applanation tonometer having a microprocessor and a prism holder combined with a molded resin prism insertable into the holder in a way that requires prism replacement before proceeding with an eye examination, the combination comprising: a. the prism being molded with an element formed to extend transversely of a direction of insertion of the prism into the holder; b. the element being deformable from an initial position to a deformed position as the prism is inserted into the holder to reach an operating position within the holder; c. the element being configured so that a first deformation of the element requires more stress than any subsequent deformation of the element; d. the holder including a strain gauge positioned to engage and deform the element as the element is inserted into the holder; e. the strain gauge being arranged to produce a signal representing the strain encountered in deforming the element as the prism is inserted into the holder; f. the microprocessor being programmed to recognize the strain gauge signal representing the first deformation of the element; g. the microprocessor being programmed to proceed with an eye examination whenever insertion of a prism into the holder causes the strain gauge to produce the signal representing the first deformation of the element; and h. the microprocessor being programmed not to proceed with an eye examination whenever insertion of a prism into the holder causes the strain gauge to produce a signal distinguishably different from the signal representing the first deformation of the element. 6. The combination of claim 5 wherein the prism has a locator projection that seats in the holder in a detented position when the prism is in its operating position. 7. The combination of claim 5 wherein the prism has a gripping tab by which the prism is inserted into the holder. 8. The combination of claim 5 wherein an optical axis of the prism intersects a reticule on a rear face of the prism and an indentation on an applanation surface of the prism. 9. A method of insuring that a molded resin prism is replaced in an applanation tonometer before each eye examination, the method comprising: a. forming prism replacements that each have a tab that is initially deformable in response to a predetermined stress; b. inserting one of the prism replacements into a prism holder of the tonometer so that the deformable tab engages a strain gauge in the holder and becomes deformed by the strain gauge as the prism is moved into an operating position within the tonometer; c. arranging the strain gauge to produce a signal representing the strain encountered in initially deforming the tab as the prism is inserted into the holder; d. transmitting the strain gauge signal to a microprocessor in the tonometer and programming the microprocessor to recognize the strain gauge signal representing the initial deformation of the tab; and e. programming the microprocessor to proceed with an eye examination only upon receiving the strain gauge signal representing the initial deformation of the tab. 10. The method of claim 9 including programming the microprocessor not to proceed with an eye examination and to indicate to the tonometer user upon receiving a strain gauge signal distinguishably different from the strain gauge signal representing initial deformation of the tab. 11. A method of insuring that an applanation tonometer prism is replaced before each examination of a pair of eyes, the method comprising: a. molding the prism of resin with a deformable element that is deformable to a predetermined extent without breaking away from the prism; b. configuring the deformable element to have an initial resistance to an initial deformation to the predetermined extent and a reduced resistance to any subsequent deformation to the predetermined extent; c. arranging a strain gauge within the tonometer to measure deformation resistance of the deformable element; d. arranging the strain gauge to produce a strain signal delivered to a microprocessor in the tonometer as representative of the deformation resistance of the deformable element; and e. programming the microprocessor to proceed with the examination of the pair of eyes only when the strain signal coincides with the strain representing the initial resistance. 12. The method of claim 11 including programming the microprocessor not to proceed with the examination of the pair of eyes and to indicate to the tonometer user when the strain signal the distinguishably differs from the strain representing the initial resistance. 13. A tonometer and applanation prism combined to ensure replacement of the prism before proceeding with examination of a pair of eyes, the combination comprising: a. the prism having a deformable element configured to have a larger resistance to an initial deformation and a smaller resistance to a subsequent deformation; b. the tonometer having a strain gauge arranged to cause deformation of the deformable element as the prism is inserted into the tonometer; c. a microprocessor in the tonometer receiving a deformation signal from the strain gauge and being programmed to recognize an initial deformation signal corresponding to the strain encountered during the initial deformation of the deformable element; and d. the microprocessor being programmed to proceed with the examination of the pair of eyes only upon receiving the initial deformation signal. 14. The combination of claim 13 wherein the deformable element is a bendable tab oriented transversely to a direction of insertion of the prism into the tonometer. 15. The combination of claim 13 wherein the deformable element is configured not to break away from the prism. 16. The combination of claim 13 wherein the prism includes a location projection that snap fits into a seated position when the prism is in an operating position within the tonometer. 17. A disposable molded resin prism usable in an applanation tonometer, the prism comprising: a. a bendable tab formed of the molded resin of the prism to extend from a body of the prism; b. the tab being configured to be bent from an initial position to a bent position as the prism is inserted into the tonometer; c. the tab being formed to survive bending without breaking off from the prism body during insertion of the prism into the tonometer and removal of the prism from the tonometer for disposal; and d. the tab being formed to have a resistance to an initial bending that is larger than any resistance of the tab to a subsequent bending. 18. The prism of claim 17 wherein the tab is formed with a thin hinge line allowing the tab to bend. 19. The prism of claim 17 wherein the tab extends from a non-optical surface of the prism body. 20. A tonometer applanation prism and prism holder comprising: the prism being molded of resin material and having a positioning projection extending approximately in a direction of insertion of the prism into the holder; the positioning projection and holder being configured to interact in a detent that engages as the prism reaches an operating position within the holder; and the detent between the positioning projection and the holder being effective when engaged to hold the prism against withdrawal from the operating position; the positioning projection is configured to flex as the positioning projection engages the detent; flexing of the positioning projection within the holder as the detent engages draws the prism into engagement with an emitter and a detector in the holder; and the positioning projection is arranged to be manually flexed to release the engaged detent, which causes the prism to spring back to a position short of the operating position.