Copied from Chat:

For buffy (who reads everything:)

I was at the RVEEH a few days ago for a continuing assessment of the uveitis. Before seeing the ophthalmologist I saw an orthoptist who ran through a series of eye tests, including the use of spectacles with interchangable lenses (phoropter?) to measure things like astigmatism, especially in the eye with severe keratoconus (which is the affected eye.) One of the instruments she used had a handle and three lenses (rough image:)

When the image I saw was relatively sharp, the orthoptist held one of the lenses of this instrument over the lens set, which changed the clarity (sharpness) of the image I saw; sometimes she would rotate the instrument keeping the same lens over the eye, which gave a different compensation. Sometimes the rotation would be about the axis of the length of the instrument (so if the instrument was horizontal as seen in the diagram, it would stay horizontal but I’d see through the lens from the opposite side. Other times she would rotate it so it was now vertical. The sharpness of the image changed by differing amounts in each case.

What’s the instrument called, and how does it work (that is, how can changing the direction of view (back-to-front instead of front-to-back) and the angle the instrument is held change the magnification of the lens?)

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What you describe sounds like using a crossed cylinder for working out amount and angle of astigmatic correction. But I’ve only ever had crossed cyls with one lens on the stick (mostly I used plus/minus 0.25D. I did have a plus/minus 0.37D too, I think). It’s a pretty old fashioned way of doing it, but sometimes keratoconics are weird to refract. Mostly I plonked a high cyl correction into the phoropter/refractor head and just turned it around until they said “Stop!”. I saw them often…we got good at seat of the pants refraction.

A historical paper:

https://sci-hub.mksa.top/10.1001/archopht.1940.00870030066009

THE JACKSON CROSSED CYLINDER

A CRITIQUE

(1940)

https://vision2020lvrc.org.hk/refractive-equipment-subjective/174-%C2%B1025d-crossed-cylinder-cc93025.html

There is a picture of the version I know here.

The other possibility would be flippers for plus/minus sphere, or with prism. I would think it more likely to be a crossed cyl though.

Ooh – this set is just gorgeous!

buffy said:

Ooh – this set is just gorgeous!

They are nice.
Tools are like that though. The very finest tools do basically the same job as the cheapest ones. However they look better, feel better and often do the job more efficiently.

I don’t trust the standard tests for astigmatism, at all. They are way too sensitive to the ocurrence of any line-like floater that passes through the field of vision. These floaters last long enough, of the order of ten to thirty seconds, to make complete rubbish of the normal tests.

The best way I know of to detect astigmatism in a way that eliminates the influence of floaters is to generate a blur circle by passing a point source of light through a very wrong (eg. +- 3 dioptres out) lens. With true asigmatism this is seen as a true ellipse. If what is seen is not a true ellipse, eg. a circle with a bight out of it or black dots or lines on it, then it’s not astigmatism, it’s floaters or something else.

Thanks buffy. I’d never heard of the crossed cylinder before; it’s a brilliant idea.

btm said:

Thanks buffy. I’d never heard of the crossed cylinder before; it’s a brilliant idea.

I used one on pretty much every patient all my practising life. They are built into the phoroptor, but perhaps she wanted to use a stronger lens than the built in one.