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(Mostly) historic scientific instruments sometimes have a telescope-like device mounted on them in order to be able to read a very fine scale accurately.

My understanding is that telescopes are used to magnify things far away, where for a magnifying eyepiece I imagine something mobile, such as a jeweller might use. So is there a word for a tubular optical instrument used to magnify things close-by?

here are a couple of examples -

Micrometer mural circle

Seagrave astronomical micrometer

  • a telescope, but backwards. – kc m Sep 1 '16 at 17:10
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    I don't think this is off-topic. It's a technical term for historic scientific instruments so it may be challenging even for a technically-inclined native speaker <cough>. Anyway, it appears to be called a "telescope"- you can find examples by renowned instrument makers of the past such as [Leeds and Northrup](ece.ut.ac.ir/classpages/S84/Electrical Measurment/82-83/Galvanometers.htm). – Spehro Pefhany Sep 1 '16 at 17:37
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    Also check reticle or reticule – Phil Sweet Sep 1 '16 at 18:35
  • @PhilSweet - this is helpful and related but SpehroPefhany provided the answer most fitting my needs (Background: I am translating a figure caption from German, which uses the German word for telescope - I have to admit it sounds wrong to me in German and English but seems to be in fact correct). thanks for adding the images btw – frederik Sep 1 '16 at 22:16
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    @MetaEd I disagree with the assessment of this question being off-topic. The 'difficulty' of the question seems in line with other questions asked on this site, and in fact has produced two answers which were not quite right. The dictionary definition of telescope just states the more common usage of a device for looking at far-away objects, so standard references are not sufficient, and I didn't come up with a clear result after googling around for 15 min or so. I got my answer now from Spehro Pefhany so it doesn't matter now, but I don't think this question was off-topic – frederik Sep 1 '16 at 22:30
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I believe that it would depend on the form of the eyepiece. If it contains a single lens it could be simply a 'lens' or a 'magnifier'. If it consists of two or more lenses it would be a 'microscope'. It would be a very specialised, low power, fixed focus microscope of use only for its intended task but it would have an object lens and an ocular lens so the optical principles would be the same as any other optical microscope. Why the original refers to it as a 'telescope' I have no idea since the optics are completely different.

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A jeweller would use a "Loupe."

These can range from handheld magnifying glasses, to ones that clip onto glasses or monocular lenses.

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It is a filar micrometer (eyepiece).

A filar micrometer is a specialized eyepiece used in astronomical telescopes for astrometry measurements, in microscopes for specimen measurements, and in alignment and surveying telescopes for measuring angles and distances on nearby objects. The word filar derives from Latin filum, meaning "a thread". It refers to the fine threads or wires used in the device.

  • Interesting answer and I enjoyed reading up about the filar micrometer. It gets very close but seems to be limited to astronomical instruments. – frederik Sep 2 '16 at 16:43
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From this and this page, it is called a eyepiece or ocular lens.

  • Similar in appearance but not quite what I was looking for. Ocular lens or eyepiece form part of the optical system of a microscope but need to work in tandem with the objective. – frederik Sep 1 '16 at 22:23

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