2. A device assembly for non-invasive determination of the intra-ocular pressure of an eye, which has a cornea bounded by a surrounding sclera, the device assembly comprising: a pressure measuring device; and a contact lens engagement piece; said pressure measuring device comprising: a pressure chamber with a flexible membrane; an external suction port; a pressure port; an engagement ring configured to be attached to the sclera; and a strain measurement means; wherein: the pressure port is in fluid contact with the pressure chamber and is configured to charge the pressure chamber with fluid to exert pressure on the cornea via the flexible membrane; the pressure chamber is equipped with said strain measurement means, said strain measurement means configured to determine the intra-ocular pressure due to the deflection of the cornea by the exerted pressure; wherein the flexible membrane is held onto the sclera by means of said engagement ring wherein the contact between the flexible membrane and the cornea is enhanced by means of contact between the sclera and the engagement ring and wherein the engagement ring is equipped with a peripheral suction channel which is in fluid contact with the external suction port; said contact lens engagement piece has a ring configured for axial alignment and engagement with the peripheral suction channel of the engagement ring and is located, in use, between the flexible membrane and the cornea and is in contact with said membrane and cornea, said contact lens engagement piece being configured to provide a hygienic separation between the cornea and the flexible membrane. a pressure measuring device; and a contact lens engagement piece; said pressure measuring device comprising: a pressure chamber with a flexible membrane; an external suction port; a pressure port; an engagement ring configured to be attached to the sclera; and a strain measurement means; a pressure chamber with a flexible membrane; an external suction port; a pressure port; an engagement ring configured to be attached to the sclera; and a strain measurement means; wherein: the pressure port is in fluid contact with the pressure chamber and is configured to charge the pressure chamber with fluid to exert pressure on the cornea via the flexible membrane; the pressure chamber is equipped with said strain measurement means, said strain measurement means configured to determine the intra-ocular pressure due to the deflection of the cornea by the exerted pressure; wherein the flexible membrane is held onto the sclera by means of said engagement ring wherein the contact between the flexible membrane and the cornea is enhanced by means of contact between the sclera and the engagement ring and wherein the engagement ring is equipped with a peripheral suction channel which is in fluid contact with the external suction port; said contact lens engagement piece has a ring configured for axial alignment and engagement with the peripheral suction channel of the engagement ring and is located, in use, between the flexible membrane and the cornea and is in contact with said membrane and cornea, said contact lens engagement piece being configured to provide a hygienic separation between the cornea and the flexible membrane. the pressure port is in fluid contact with the pressure chamber and is configured to charge the pressure chamber with fluid to exert pressure on the cornea via the flexible membrane; the pressure chamber is equipped with said strain measurement means, said strain measurement means configured to determine the intra-ocular pressure due to the deflection of the cornea by the exerted pressure; wherein the flexible membrane is held onto the sclera by means of said engagement ring wherein the contact between the flexible membrane and the cornea is enhanced by means of contact between the sclera and the engagement ring and wherein the engagement ring is equipped with a peripheral suction channel which is in fluid contact with the external suction port; said contact lens engagement piece has a ring configured for axial alignment and engagement with the peripheral suction channel of the engagement ring and is located, in use, between the flexible membrane and the cornea and is in contact with said membrane and cornea, said contact lens engagement piece being configured to provide a hygienic separation between the cornea and the flexible membrane.