Dear John,
>>
I am trying to quantify axial distortion of fluorescent polystyrene
beads (10 um). Is anyone aware of an aqueous or liquid medium with a
refractive index >1.6 which will not dissolve polystyrene on contact?
>>

Cargille makes a wide range of immersion oils which should be safe with
polystyrene.  You can even get calibrated refraction index series, which I
have used for determining the approximate refractive index of unknown
plastics (when you can't see the edges in full field, the RI matches).  These
kits are not cheap, but I think you get what you pay for.

I don't have the company address at my fingertips, but I can dig it out for
you if you need more than the name.

I have no affiliation and so on...

David Carter
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General Manager
Meridian Instruments
Slip slidin' away
2310 Science Parkway
Okemos, MI 48864