Mapping the surface of the eye.
Mapping the surface of the eye.
Keratometers measure the radius of curvature of the anterior (front) corneal surface of the eye.
They should permit the quick and convenient measurement of the diameter of the cornea, which allows the practitioner to judge the volume of the eyeball. These instruments were particularly valuable when prescribing and fitting spectacles as the lens vertex and the corneal vertex must be in harmony. More latterly however they have been used mainly by contact lens practitioners.
The first keratometer
Jesse Ramsden (1735-1800) was the first English optician to produce a keratometer, expressly for the purpose of 'proving' Kepler's theory that accommodation of the eye was due to a change in corneal curvature. Ramsden threw it away when he discovered the sad truth. His instrument was apparently based on Ole Rømer's heliotrope, a split mirrored device for measuring the sun. The first proper keratometer, however, was produced in Paris in 1728 though it could also measure various other dimensions of the eye (for instance the anterior chamber) so it was truly an 'ophthalmometer' in a way that many of the keratometers that subsequently bore the ophthalmometer name were not.
Our museum's oldest keratometer is the Pfister & Streit Ophthalmometer, a Swiss keratometer from 1904 based on the classic design of Javal and Schiøtz (1881). Made of black metal, with a brass curving arm, the light units are propelled by a revolving balloon-shaped wooden knob. The sliding tripod stand is fixed to a wooden base with a head rest and chin rest adjustable for height. There is also a black metal swinging occluder and bakelite electical fittings which may have been added later.
The Sutcliffe New Keratometer was a heavy beast on an iron tripod stand. Developing out of the Chambers-Innskeep (Chicago) model of 1899, it featured a head rest, single eyepiece, astigmatism dial and lamp unit on a swivelling base. Our example of this instrument, shown on the right, is from circa 1907.
The third object is a slightly later development of the same instrument, dating from circa 1910. These instruments were designed by J. H. Sutcliffe, founder of the museum. They must have taken up a lot of space in the consulting room.
Sutcliffe's keratometers improved upon the Javal-Schiøtz designs up to that date in that both meridians could be measured simultaneously. It did this by means of prisms that could be moved parallel to the instrument's axis. In this way it countered any error that might occur if the patient's eye moved during the examination.
Look at the next instrument illustrated and you'll see the Sutcliffe Self-Recording Keratometer. It has a distinctively 'modern' look with its triangular base. There is a central spindle and revolving cylinder plus a brass circular scale marked in dioptric and radial units (c.1907-9).
The large parabolic bowl keratometer is an example of another commonly found type. It was made in America by F. A. Hardy & Co. and comprises black painted metal with a polished brass stem, scale and draw tubes. The separate base has an adjustable wooden chin rest (on a brass spring ratchet), leather padding at forehead level, two swing-over occluders and four two-pin power fittings at each corner. Like several of our keratometers, this item was formerly in the Keeler Collection, displayed at Windsor.
By contrast Dr. Reid's Pocket Ophthalmometer is a miniature keratometer by Kelvin & James White Ltd, just 10cm long, in black metal and brass with a small ivory scale and milled edge wheel to adjust the pointer mechanism. This example was formerly used in the Glasgow Eye Infirmary and we would love to hear from anyone who can tell us more about its date and designer.
Probably the best known instrument in the post war period was still the Javal-Schiøtz Ophthalmometer, now known in the business simply as a 'Javal', which was available under the name of various suppliers including Haag-Streit and Woodlyn. In 2003 the Museum was given the model previously used at the Stepping Hill Hospital in Stockport.
A keratometer provides readings of corneal curvature in dioptrical values, but since contact lenses were ordered and manufactured on the basis of the radius curvature (in millimetres), a conversion was always needed. Here is a revolving cardboard conversion chart, double sided (for power readings 36.00-43.87 or 44-52.00) within a three-part folder, including instructions for the Bausch & Lomb Keratometer (1960s).
Topographic modelling system
More recent equipment in the museum tends to be big. Sometimes we can only collect part of it, as in this instance.
The object is a computerised videokeratoscope / topographer with laser aperture and would have come originally with an attached computer to enhance its images for easier and more accurate diagnoses. You could use it to collect information about the shape of the cornea and its refractive characteristics. Pictorial records of patient examinations could be stored in the optional Bernoulli drive that was considered to have a huge capacity of 90MB. Made by Computed Anatomy Inc around 1989-1993 the TMS-1 was the first of a popular international range and its mapping function was based on the Corneal Modelling System (CMS) pioneered at the New York Eye and Ear Infirmary.
Even though computer technology has allowed keratoscopy to become more widely accepted and more clinically friendly, the basic principles underlying the new technology are the same as those articulated by Gullstrand a century before. The example illustrated is a late model of its type and you're looking at the part the operator would have used. The integral monitor was for checking the alignment of the instrument with the patient's eye whilst the patient attempted to fixate on a blinking white light. A cone reflected 25 concentric rings on to the surface of the eye where curvature measurements would be taken automatically. The cone could be replaced with one containing 31 rings for the purpose of precise fitting of RGP contact lenses. This particular device was used by a Harley Street ophthalmic surgeon. It is to be hoped that his consulting rooms were spacious since practitioners were advised to place the unit well away from other instruments to avoid distractions during an examination. A low power laser beam was used to position the patient's corneal axis. For safety's sake the instrument had an automatic cut-out after sixty seconds but it was reckoned that normal examination should require no more than fifteen seconds.