GB2308462A - Tonometer and fixation device - Google Patents

Abstract

An instrument for measuring a pressure contained by an elastic membrane, which instrument comprises means eg. disc 2 of cup 5 for applying a first force to flatten an area of membrane, means eg. a piston acting on a fluid 7 for applying a second force within the flattened area of the membrane and means for determining the amount of second force required to counteract the pressure contained by the membrane. The instrument and method may be used for measuring intraocular pressure of an eye 3. A fixation device (not shown) for a tonometer having an applanating face, changes in appearance to a subject when an area of the corneal surface of the eye of the subject is flattened by the applanating face. The fixation device maybe in fluid container 6 behind cup 5.

Description

TONOMETER The present invention relates to an instrument for measuring a pressure inside a cavity bound by an elastic membrane and, more particularly, to an instrument for measuring the intraocular pressure of the fluid in the anterior chamber of a human eye.

The intraocular pressure ("IOP") is the pressure of the fluid in the anterior chamber of an eye. IOP varies between individual eyes and factors including time of day, eye health and medication. If IOP falls outside a safe range, the eye's sight may be permanently reduced. This can happen to one person in about fifty, normally experiencing no symptoms before sight is permanently lost. Glaucoma is a principle eye disease associated with raised IOP.

IOP is, therefore, estimated routinely. The process is commonly known as tonometry, i.e. the use of a tonometer, and should ideally give results repeatable to within +5 . A tonometer operates by assessing the mechanical deformation of the eye shape caused by applying a known force or by finding the force that produces a given mechanical deformation of the eye's shape. The deformation indents the eye or decreases part of its surface curvature. This method is known as applanation if measurement is related principly to the extent of flattening that the instrument produces on the cornea's central front surface.

Tonometers using Goldmann applanation, the standard against which readings of IOP are evaluated, have most commonly been used. Goldmann-type tonometry, however, has several significant inherent sources of error and these are discussed in detail in an article entitled ',So;Lirces of Error with Use of Goldmann-type Tonometer" by M. Whitacre and R.

Stein, Survey of Ophthalmology, volume 38, number 1, July August 1993, pages 1 to 30. These inherent sources of error arise from determination of lOP by measurement of a force that presses a flat surface against the eye to flatten, or applanate, a known area and include: (a) Surface tension of tear fluid. The pre corneal tear film gives rise to indeterminate, complex, varying, hydrostatic forces acting on the applanating surface, thereby adding unknown errors to the pressure measurement.

(b) Variation in structural parameters of the cornea, including thickness, curvature, rigidity and superficial hardness. Variation in each of these parameters causes variation in the force that flattens a corneal area. Furthermore, these structural parameters vary at different points on each cornea, thereby leading to a significant source of error in the pressure readings.

(c) Observer bias (e.g. on interpreting the tonometer's display to determine the result) renders measurement subjective, rather than by more accurate objective means.

(d) The applanating surface of a Goldmann-type tonometer is attached to a lever that can rotate in a plane perpendicular to the applanating surface.

Applanating force is applied as a moment of force about the pivot through the lever to the applanating surface. If this surface is positioned askew on the cornea, a false moment of applanating force is measured, thereby leading to an inaccurate pressure reading.

(e) A fluorescent material is often added to a patient's eyes in Goldmann-type tonometry in order to assist the operator, who sees the fluorescence, and judges the pressure measurement by the position of the fluorescence. However, the amount of fluorescence can vary due to variation in the physical chemistry of different eyes, the same eye in different circumstances and the amount of fluorescent material present, thereby leading to variations in the measurement of IOP.

Other tonometers have been designed to work on a number of different principles, including: (a) Measurement of the indentation of the cornea (as opposed to flattening of the cornea) by a fixed force, or of a force producing a fixed indentation of the cornea (indentation).

(b) Measurement of the least pressure of a jet of air expelled from a nozzle at a fixed position from the cornea that is required to flatten the surface of the cornea (air puff).

(c) Measurement of the least pressure of gas that escapes between two surfaces pushed together by a force that flattens a certain area of the cornea (pneumatonometry).

(d) Measurement of an electrical signal given by a sensor coming into and out of contact with the eye (mechanical transduction).

However, these methods can give less accurate, less repeatable and less reliable results than even Goldmann-type tonometry. For example, indenting, rather than just flattening, the eye raises the IOP by an unknown amount, thereby resulting in a yet further source of error.

Measurements using a jet of air are particularly affected by variations in the structural parameters of the cornea and air turbulence may also cause spurious variations. Use of pneumatonometery and mechanical transduction can both lead to errors arising from hydrostatic forces of tear fluid and variations in the structural parameters of the cornea whilst mechanical transduction has a further source of error arising from variations in contact velocity. Measurements using a jet of air and mechanical transduction, in particular, give poor repeatability. These are some reasons why these other methods of tonometry have not found significant use in ophthalmological diagnosis and treatment.

The need for a more reliable tonometer has been heightened by the gradual acceptance that glaucoma often occurs with apparently normal tonometry results, whilst it is not always signified by high pressure readings. In particular, because of inherent variability of biometric material, borderline-high IOP apparently damages some eyes but not others, rendering a reliable means of measuring IOP essential. It is envisaged that the need for a reliable tonometer will also increase with the growth in popularity of surgery to alter corneal shape. This type of surgery is used to adjust the focal power of the eye, so as to reduce the need for spectacles or contact lenses, but greatly increases the variation in structural parameters of the cornea and, hence, the errors of tonometry by current methods. Such surgery may become widespread, confounding the diagnosis and treatment of glaucoma. Furthermore, such surgery often requires subsequent monitoring of TOP because this can be affected by post-operative medication.

As mentioned above, the IOP of a human eye to be measured is contained by the cornea which has a significant rigidity. Any force applied to the surface of the cornea must, therefore, act against this rigidity in addition to acting against the contained pressure. Because the rigidity is a complex and indeterminate variable, it can be a significant source of error in all prior art tonometry.

Accuracy in measurement of the contained pressure, thus, requires minimal deformation of this membrane.

According to the present invention, there is provided an instrument for measuring a pressure contained by an elastic membrane, which instrument comprises means for applying a first force to flatten an area of membrane, means for applying a second force within the flattened area of the membrane and means for determining the amount of second force required to counteract the pressure contained by the membrane. Because the measuring force is separate from the flattening force, errors arising from any variations in the structural parameters of the elastic membrane are minimised.

Variations in the positioning of the flattening force on the elastic membrane will not significantly affect the measurements. Furthermore, observer or operator bias does not influence the result so that the present invention provides an objective measurement of the pressure which is generally more accurate than previous, subjective methods.

The first and second forces are separately controllable.

Preferably, the first force is substantially constant.

Preferably, the means for applying the first force comprises a flat surface which is pressed, in use, against an area of the membrane.

Preferably, the means for applying the second force comprises a volume of pressurised fluid which acts, in use, against the flattened area of the membrane.

More preferably, the flat surface comprises one or more holes through which the second force is applied, in use. Because the measuring force is applied through the one or more holes in the flat surface to counteract the pressure contained by the elastic membrane, any hydrostatic forces, for example, arising from the surface tension of tear fluid in the case of an eye, act against the flattening force only, so that sources of error arising from any such hydrostatic forces are, again, minimised.

Preferably, the flat surface is provided at one end of a lever, the lever being mounted for rotation about a fulcrum and the first force being applied, in use, as a moment of force about the fulcrum, More preferably, actuating means operable to generate the moment of force about the fulcrum comprises a solenoid.

Preferably, the volume of fluid is contained in a fluid container which is in fluid communication with the one or more holes.

More preferably, a piston is used to adjust the fluid pressure within the container.

Preferably, the determining means comprises a microprocessor.

According to another aspect of the present invention, there is provided a fixation device for use in a tonometer having an applanating face, which device changes in appearance to a subject when an area of the corneal surface of an eye of the subject is flattened by the applanating face.

Preferably, the fixation device comprises a pair of translucent targets and a converging lens which are centred on, and perpendicular to, the subject's visual axis, in use, the positions and sizes of the targets being such that the subject sees one target changing in colour on applanation of a central area of the corneal surface of the eye. This device may be used with the instrument of the present invention or any other tonometer and provides a more accurate means for correctly positioning the flat face or applanating face on the elastic membrane or corneal surface of the eye than previous methods.

According to a further aspect of the present invention, there is provided a method for measuring a pressure contained by an elastic membrane, which method comprises applying a first force to flatten an area of membrane, applying a second force within the flattened area of membrane and determining the amount of second force that is required to counteract the pressure contained by the membrane. Again, the use of separate flattening and measuring forces minimises or eliminates many of the sources of error arising in prior art methods, including minimisation of errors due to variations in the structural parameters of the elastic membrane, due to variations in the positioning of the measuring force, and due to observer/operator bias.

Preferably, the first force is applied by pressing a flat surface against an area of the membrane.

More preferably, pressing of the flat surface against the area of the membrane is electromechanically controlled.

Preferably, the second force is applied by a piston acting on a fluid pressing against the flattened area of the membrane.

More preferably, the piston is driven by a magnetic force generated by a motor.

More preferably, the second force is applied through one or more holes in the flat surface. Again, any hydrostatic forces (for example, due to surface tension of tear fluid in the case of an eye) act against the flattening force and not against the measuring force so that this source of error found in prior art tonometers is minimised.

Preferably, the method further comprises analysing fluid pressure readings of the fluid pressing against the flattened area of the membrane and the rate of change of fluid pressure.

More preferably, the pressure to be measured is determined as the least amount of second force that is required to permit a small amount of fluid to seep through the one or more holes in the flat surface between the flat surface and the membrane.

More preferably, the seepage of the fluid through the one or more holes is detected as a decrease in the rate of increase of fluid pressure.

The instrument and method of the present invention may be used for measuring intraocular pressure, in particular, the intraocular pressure of a human eye.

In order to illustrate the features and advantages of the present invention, the present invention will now be further described, by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows a side view of the mechanical arrangement of a tonometer according to the present invention; Figure 2 shows a front view of the mechanical arrangement of Figure 1; and Figure 3 shows a detailed view of the applanating head of the tonometer of Figures 1 and 2.

With reference to Figures 1 and 2 of the accompanying drawings, an example of an instrument according to the present invention comprises an elongate, hollow handle 1 which encases a transparent, hollow, perspex, openended, L-shaped fluid container 6. One end of the long arm of the L-shaped fluid container 6 runs into a cylindrical reservoir 12 and the other end of the L-shaped fluid container 6 is formed integrally with one end of the short arm. The short arm of the L-shaped fluid container 6 is provided with a replaceable, lightweight, transparent cup 5 push-fitted on to the other, free, end thereof in a fluidtight manner.

The short arm of the L-shaped fluid container 6, with the cup 5, in place, extends through an opening 24 in the handle 1 during use, and is long enough to clear facial structures, such as the lower eyelashes, during applanation.

The space 7 within the fluid container 6, is continuous with the space within the cup 5 and reservoir 12 and is filled with a fluid (not shown). The closed end of the cup 5 comprises a flat, transparent perspex disc 2 which is approximately 5mm in diameter and acts as an applanating surface. The disc 2 has one or more circular holes 4 in the central 1 - 2mm thereof and the fluid in the fluid container 6 can only leave the space 7 through these one or more holes 4.

The one or more circular holes 4, in the disc 2, have rounded shoulders to avoid a sharp edge that could endanger the surface of the cornea and to prevent a temporary decrease in corneal comfort and function.

Replacement cups 5 may be stacked and dispensed from a spring-loaded compartment (not shown) within the handle 1 of the instrument and the cup 5 may be discarded or disinfected after use.

The reservoir 12 has a solid elongate extension 20 with a steel armature 21 on the opposite side to the fluid container 6, and the outer surface of the reservoir 12 is attached to inner opposing side walls of the handle 1 by means of a pair of undamped pivots 8 so that the combined mass of the fluid container 6, cup 5, reservoir 12, solid extension 20 and steel armature 21, is balanced about the pivots 8. The reservoir 12 is so pivoted as to allow the fluid container 6 to rotate only within a single plane, which is perpendicular to the flat surface of the disc 2, and about an axis which is perpendicular to both the longitudinal axes of the long and short arms of the fluid container 6, such that the short arm of the fluid container 6 with the cup 5 in place is able to move backwards and forwards through the opening 24 of the handle 1.The pivots 8 are undamped to provide minimal friction so that the operator need not hold the tonometer handle perfectly stationary in relation to the subject in order to achieve a steady applanation of the corneal surface.

When the instrument is not in use, the short arm of the fluid container 6 is parked against a cushion 9 by means of weak pressure from a spring 10 between the long arm of the fluid container 6 and an inner wall of the handle 1.

Forward travel of the short arm of the fluid container 6 is limited by a cushioned buffer 25 containing a pressureoperated switch 11.

The solid extension 20 extends along the longitudinal axis of the long arm of the fluid container 6, and in the opposite direction thereto. The free end of the solid extension 20, distal to reservoir 12, bears the steel armature 21 which extends perpendicularly away from the solid extension 20, along an axis which is parallel to the longitudinal axis of the short arm of the fluid container 6 and in the opposite direction thereto. The steel armature 21 is attached to the solid extension 20 at a damped sprung coupling 22 which acts as a tension transducer.

The steel armature 21 is encircled by a solenoid 23 and applanating force is applied to the armature 21 by the solenoid 23. This causes the short arm of the fluid container 6 to rotate about the pivots 8 in a forwards direction through the opening 24 of the handle I so that the disc 2 of the cup 5, in turn, presses against and flattens an area of the corneal surface of a patient's eye. Thus, applanating force is applied as a moment of force about the pivots 8, and the applanating force is stabilised by controlling the current in the solenoid 23 through feedback from the transducer 22. The gain of this force feedback loop may be modulated. This ensures that a more or less constant applanating pressure is applied to the corneal surface 3 of the eye, even when the instrument is hand-held.

The reservoir 12 contains a barometer or pressure transducer 13 with an electrical signal output and a piston 14 for altering the pressure of the fluid in the fluid space 7. The reservoir 12 is vented at a position 15 behind the piston 14 to facilitate movement of the piston 14. The piston 14 bears a co-axial, free-wheeling, magnetic disc (not shown) which may be mounted on a low-friction ballbearing race (not shown) on the axis of the piston 14. A lightweight collar 16 is provided in a screw thread 17 which extends around the outer surface of the reservoir 12 at one end of the reservoir 12 and about the longitudinal axis thereof. The lightweight collar 16 bears steel studs 18 extending outwardly therefrom to enable the collar 16 to be turned as the armature of a linear motor 19.The piston 14 is, thus, able to be driven along its axis by magnetic coupling between the magnetic disc and collar 16.

Interruptor switches (not shown) at the ends of the screw thread 7 are provided to limit the extent of travel of the collar 16.

Either a liquid or gaseous fluid may be used to fill the reservoir 12 and fluid space 7. The fluid, if liquid, should be non-toxic to the eye and periocular surfaces and should be of low viscosity. A light oil, such as sunflower oil, would be suitable if pure and has the further advantage of being inexpensive and readily available. Any other suitable alternatives may, however, be used. In the case where air is used as the fluid, mechanisms for re-filling and sterilising the fluid space are not needed and the use of air also permits the disc 2 of the cup 5 to have just a single hole, centrally, of approximately l-1.5mm in diameter, this being easier to produce and clean than a disc 2 with more, or smaller, holes.

The instrument shown in the drawings is designed for fully automated measurement of TOP. Switching the instrument on operates the solenoid 23 and its transducer feedback control to give an applanating force of approximately 1.5gm. The pulling action of the solenoid 23 on the steel armature 21 causes the short arm of the fluid container 6 to rotate forwards through the opening 24 in the handle 1 until the fluid container 6 reaches its forward limit of travel. This closes the buffer switch 11, signalling to a microprocessor (not shown) of the instrument to clear readouts and initialise circuits (not shown) serving the pressure transducer 13 and motor 19.

After topical anaesthesia, part of the corneal surface 3 of the eye is applanated by applying the disc 2 of the cup 5 thereto. A fixation device (not shown) is provided inside the short arm of the fluid container 6, just behind the cup 5, to guide the patient. This device consists of two translucent visual targets (not shown) which differ in colour and can be seen by the patient through the transparent material of the fluid container 6 and cup 5, and a converging lens, all of which are centred on and are perpendicular to the patient's visual axis during central corneal applanation. The positions of the visual targets are arranged so that the patient focuses the nearer target through the lens immediately before applanation and the further target through the lens during applanation.The change in focus from the nearer target to the further target occurs as a result of neutralisation of the focusing power of the cornea during applanation and the sizes of the visual targets are arranged to give an identical apparent target size during applanation, as indicated by a simple colour change. The fixation device facilitates centration of the disc 2 on the corneal surface 3 of the patient's eye.

The disc 2 of the cup 5 is, preferably, positioned centrally on the corneal surface 3 of the eye to give an optimum applanated area of diameter 3.06mm. In previous methods of tonometry, including Goldmann-type and pneumatonometry, it was necessary for such a precise, set area of the corneal surface of the eye to be applanated for empirical reasons. At a smaller area, for example, error due to corneal compressibility and the possibility of corneal damage may be significant, whilst applanation of a larger area makes the instrument more difficult to use. In the case of the present invent ion, however, there is no technical need for the applanated area to be set, precision in this respect not being critical.

Where such a fixation device is used, the cups 5 may be produced with at least three different lengths. The longer and shorter versions of the cup 5 may be selected to enhance focus of the fixation target in, respectively, highly hypermetropic and highly myopic subjects.

Applanation may be viewed by the operator directly and in comfort from the side. The buffer switch 11 is only closed when the fluid container 6 is at its forward limit of travel so that, when applanation takes place by applying the disc 2 of the cup 5 against the corneal surface 3 of the eye, the corneal surface 3 of the eye pushes the fluid container 6 back a little from its forward limit of travel to re-open the buffer switch 11. This signals to the microprocessor (not shown) to start the motor 19 and to monitor the barometer 13.

The fluid in the reservoir 12 and fluid space 7 is adjusted, for example, in a range of approximately 2mm Hg and at a frequency of approximately two to three times a second (or at any other suitable range or at any other suitable frequency). As the pressure of the fluid in the reservoir 12 and fluid space 7 is increased by action of the piston 14, pressure builds up at the one or more holes 4 in the disc 2 which is being pressed against the corneal surface 3 of the eye.The applanating force of the disc 2 against the corneal surface 3 of the eye prevents the pressure of the fluid in the reservoir 12 and fluid space 7 from repelling the disc 2 from the eye, even at the highest values of IOP and with a hole area of just over limit Fluid pressure readings are analysed by the microprocessor at frequent, regular, intervals for example, in the order of a millisecond. Readings are discarded if differing significantly from the mean of previous readings.

The rate of change of the fluid pressure in the reservoir 12 and fluid space 7, dp/dt, is also monitored by the microprocessor via the pressure transducer 13. IOP is determined as the minimum pressure of fluid in the reservoir 12 and fluid space 7 which overcomes IOP to permit fluid to escape between the flattened corneal surface 3 and the applanating face of the disc 2. Seepage of fluid from the one or more holes in the disc 2 causes the function dp/dt to drop slightly and it is at this point that the microprocessor slows the motor 19 and IOP is first read.

Because the pressure of the fluid in the reservoir 12 and fluid space 7 is not proportional to the pressure of the disc 2 against the corneal surface 3, many of the sources of error associated with prior art methods of tonometry, including surface tension of tear fluid, variation in structural parameters of the cornea and observer bias, are minimised or eliminated.

IOP varies substantially in a normal eye with the pulsing of blood pressure and the microprocessor of the instrument shown in the drawings also identifies and analyses cycling of dp/dt due to the vascular pulse. Dp/dt changes between positive and negative twice with each heartbeat, as the vascular pulse in IOP triggers a sudden change in the dp/dt. If rising IOP is more frequent, the speed of the motor 19 is increased to increase the rate of increase in fluid pressure, and vice versa. By increasing or decreasing the speed of the motor 19 in this way, periods of rise and fall of IOP due to vascular pulse may be achieved of similar duration. Changes in dp/dt due to vascular pulse may be governed for example, through barometer readings fed back for software control of the motor 19.Each positive value, could, for example, be governed to be the negative of the preceding negative value except for the initial dp/dt, which could be fast, approximately l0mmHg/sec, to save time before the first inflection. When periods of rising and falling vascular pulse IOP are of similar duration, their median values of fluid pressure are stored.

An indicator lamp (not shown) may be provided to inform the operator that readings of IOP are being obtained and measurement continues until the instrument is withdrawn from the patient's eye. Because the solenoid 23 still operates when the instrument is withdrawn from the eye, the fluid container 6 is returned to its forward limit of travel, thereby closing the buffer switch 11. This signals to the microprocessor to reverse the motor 19, switch off the solenoid 23 and analyse the stored readings. Analysis preferably takes the longest series of readings with less than 20E variation from median values and their mean is displayed on a readout, labelled, for example, "intraocular pressure". Any other suitable analysis and display of readings may, alternatively, be used.The number of readings in each series divided by their variance may also be shown on a separate display as the "confidence" index.

Further information about IOP could be obtained, for example, by setting the microprocessor to find the pressure level at which rising pressure periods of vascular pulse IOP are not equal to, but given proportions longer than, periods of falling pressure. The microprocessor is able to locate various points within the intraocular pressure pulse cycle and to analyse fluid pressure readings at these points in order to permit analysis of intraocular fluid dynamics.

Measurements of IOP are taken at more or less constant applanating pressure, which is monitored through the transducer at coupling 22. However, because the fluid pressure is separate from the applanating force, moderate variations in applanating pressure are not critical and do not generally affect readings of IOP.

A timer (not shown) may be provided to shut off power to the instrument if the buffer switch 11 is not operated for 5 minutes and a beeper (not shown) may be provided to signal inadequate battery power.

The instrument of the present invention provides repeatability of results which are substantially better than +58 if the instrument is produced to a reasonable yet economic standard.

Although the invention has been described for use in measuring the IOP contained by the corneal surface of an eye, the instrument may also be used to measure the pressure contained by the conjunctival (white) surface of the eye or in any other relevant field where a pressure inside an elastic cavity is to be measured.

Claims (56)

1. An instrument for measuring a pressure contained by an elastic membrane, which instrument comprises means for applying a first force to flatten an area of membrane, means for applying a second force within the flattened area of the membrane and means for determining the amount of second force required to counteract the pressure contained by the membrane.

2. An instrument according to Claim 1, wherein the first and second forces are separately controllable.

3. An instrument according to Claim 1 or 2, wherein the first force is substantially constant.

4. An instrument according to any of Claims 1 to 3, wherein the means for applying the first force comprises a flat surface which is pressed, in use, against an area of the membrane.

5. An instrument according to any of Claims 1 to 3, wherein the means for applying the second force comprises a volume of pressurised fluid which acts, in use, within the flattened area of the membrane.

6. An instrument according to Claim 4 and 5, wherein the flat surface comprises one or more holes through which the second force is applied, in use.

7. An instrument according to Claim 6, wherein the one or more holes have rounded shoulders.

8. An instrument according to Claim 6 or 7, wherein the flat surface is provided at one end of a lever, the lever being mounted for rotation about a fulcrum and the first force being applied, in use, as a moment of force about the fulcrum.

9. An instrument according to Claim 8, wherein the lever comprises a first lever arm extending from the fulcrum to the flat surface and a second lever arm extending from the fulcrum to an actuating means, the actuating means being operable to generate the moment of force about the fulcrum.

10. An instrument according to Claim 8 or 9, wherein the lever is mounted for rotation on a pair of undamped pivots.

11. An instrument according to Claim 9 or 10, wherein the actuating means comprises a solenoid.

12. An instrument according to Claim 11, further comprising a tension transducer in the second lever arm and control means for controlling the solenoid via a feed-back loop acting through the tension transducer.

13. An instrument according to Claim 11 or 12, wherein the solenoid acts, in use, on a perpendicular extension of the second lever arm.

14. An instrument according to any one of Claims 9 to 13, wherein the first lever arm comprises a container for the volume of fluid, the fluid container being in fluid communication with the one or more holes.

15. An instrument according to any one of Claims 9 to 14, wherein the flat surface forms the end of a cup, the cup being removably mountable on the first lever arm.

16. An instrument according to Claim 15, wherein the cup is mountable on a perpendicular extension of the first lever arm.

17. An instrument according to any one of Claims 14 to 16, wherein the fluid container is provided with a piston for adjusting fluid pressure within the container.

18. An instrument according to Claim 17, wherein the piston is provided with a coaxial, free-wheeling, magnetic disc on the outer surface thereof, the magnetic disc being magnetically coupled, in use, to a driving armature of a motor.

19. An instrument according to Claim 18, wherein the armature of the motor comprises a collar screw-threaded onto an outer surface of the first lever arm.

20. An instrument according to Claim 18 or 19, further comprising means for controlling the speed of the motor.

21. An instrument according to any of Claims 14 to 20, wherein the fluid container further comprises a pressure transducer for measuring the pressure of fluid in the fluid container.

22. An instrument according to Claim 21, further comprising means for monitoring the fluid pressure readings from the pressure transducer.

23. An instrument according to any one of Claims 5 to 22, wherein the fluid comprises a liquid.

24. An instrument according to Claim 23, wherein the liquid comprises sunflower oil.

25. A fluid according to any one of Claims 5 to 22, wherein the fluid comprises air.

26. An instrument according to any one of the preceding claims wherein the determining means comprises a microprocessor.

27. An instrument according to Claim 26 when dependent on Claims 12, 20 and 27, wherein the microprocessor comprises the solenoid control, motor control and pressure transducer monitoring means.

28. An instrument according to Claim 27, further comprising a buffer switch which signals to the microprocessor, in use, to clear read-outs and initialise circuits serving the solenoid and motor control means at the first lever arm's forward limit of travel.

29. An instrument according to Claim 28, wherein the microprocessor further comprises differentiator means for measuring the rate of change of fluid pressure.

30. An instrument according to Claim 29, wherein the microprocessor is operable to slow the motor and analyse pressure readings upon an indication of a decrease in the rate of increase of fluid pressure by the differentiator means, the decrease in the rate of increase of fluid pressure being indicative of fluid seeping through the one or more holes.

31. An instrument according to any one of the preceding claims, wherein the membrane comprises the corneal surface of an eye and the pressure to be measured is intraocular pressure.

32. An instrument according to Claim 31, when dependent on any of Claims 26 to 31, wherein the microprocessor is operable to analyse and compensate for intraocular pressure due to vascular pulse.

33. An instrument according to Claim 31 or 32, further comprising a fixation device which changes in appearance to a subject when the flat surface of the instrument is in a desired position on the corneal surface of the eye.

34. An instrument according to Claim 33, wherein the fixation device comprises a pair of translucent targets and a converging lens, centred on and perpendicular to a subject's visual axis, in use, the positions and sizes of the targets being such that the subject sees a colour change during flattening of an area of membrane by the flat surface, in use.

35. An instrument according to any one of Claims 31 to 34, when dependent on any of Claims 26 to 30, wherein the microprocessor is operable to analyse fluid pressure readings at known points in the intraocular pressure pulse cycle.

36. An instrument according to any one of the preceding claims, wherein the instrument is hand-held or standmounted, in use.

37. An instrument according to any one of the preceding claims for use in measuring intraocular pressure.

38. A tonometer which can be used for measuring the intraocular pressure of the eye by determining the force that counteracts intraocular pressure on an applanated cornea, the counteracting force being applied and adjusted separately from the applanating force.

39. A fixation device for use in a tonometer having an applanating face, which device changes in appearance to a subject when an area of the corneal surface of an eye of the subject is flattened by the applanating face.

40. A fixation device according to Claim 39, wherein the fixation device comprises a pair of translucent targets and a converging lens which are centred on, and perpendicular to, the subject's visual axis, in use, the positions and sizes of the targets being such that the subject sees a colour change during applanation of an area of the corneal surface of an eye.

41. A fixation device according to Claim 39 or 40 for use with an instrument according to any of Claims 1 to 37 or a tonometer according to Claim 38.

42. A method for measuring a pressure contained by an elastic membrane, which method comprises applying a first force to flatten an area of membrane, applying a second force within the flattened area of the membrane and determining the amount of second force that is required to counteract the pressure contained by the membrane.

43. A method according to Claim 42, wherein the first force is applied separately from the second force.

44. A method according to Claim 42 or 43, wherein the first force is applied by pressing a flat surface against an area of the membrane.

45. A method according to Claim 44, wherein pressing of the flat surface against the area of membrane is electromechanically controlled.

46. A method according to any of Claims 42 to 45, wherein the second force is applied by a piston acting on a fluid pressing against the membrane within the flattened area of the membrane.

47. A method according to Claim 46, wherein the piston is driven by a magnetic force generated by a motor.

48. A method according to Claim 44 or 45 and Claim 46 or 47, wherein the second force is applied through one or more holes in the flat surface.

49. A method according to any one of Claims 42 to 48, further comprising analysing fluid pressure readings of the fluid pressing against the membrane and the rate of change of fluid pressure.

50. A method according to Claim 48 or 49, wherein the pressure to be measured is determined as the least amount of second force that is required to permit a small amount of fluid to seep through the one or more holes in the flat surface between the flat surface and the flattened area of the membrane.

51. A method according to Claim 50, wherein seepage of fluid through the one or more holes is detected as a decrease in the rate of increase of fluid pressure.

52. A method according to any one of Claims 44 to 50, for measuring intraocular pressure.

53. An instrument substantially as hereinbefore described, with reference to and as shown in Figures 1 to 3 of the accompanying drawings.

54. A fixation device substantially as hereinbefore described.

55. A method substantially as hereinbefore described, with reference to Figures 1 to 3 of the accompanying drawings.

56. Any novel feature or combination of features described herein.