What is the prism on my sextant?

My friend recently bought a German sextant, from 1924. The sextant comes with one, optional, prism, which we can't figure out how to use.

Anyone who knows what the prism is for?

Thanks Jonas

Some years ago I had a 'Hezzanith ' sextant which had a prism as an optional extra. I never had such a prism so I have no experience as to how it worked. However I have in front of me as I write a copy of the instruction booklet which came with it (priced 1shilling & 6 pence)!. Two prisms as attachments are described . First is a 'Wollaston prism' which divided the light from a star into two separated beams 16 mins apart and in observations the horizon is brought midway between the two images. This is said to be advantagous when the horizon is not well defined. The prism was attached between the horizon glass and the index mirror so would not correct sextant tilt in any way. The second is a 'Nicol's prism'to be fitted in front of the telescope to polarize the light from the horizon. This was simply a means for cutting out horizon glare. The third is a simple 30.00 deg prism fitted half in front of the telescope as a device to check centering error and calibration of the arc. Other fixed angles were availiable. So no help here I'm afraid to help correct tilt. However it seems to me that if a ray splitting prism were mounted in front of the horizon glass, it could be used to image the horizon from two sideways directions, say 90 deg apart, two superimposed horizons would now appear in the index glass. These images would separate as the sextant was rocked and only coincide if the sextant were exactly vertical. The image of the sun etc would then be superimposed on these merged horizons in the normal way but without the need to rock the sextant during the observation. I don't know if this is what you had in mind or if this idea has been exploited anywhere. I hope this helps your inquiry