How does a refractometer work?

[parthenope]

Could anyone explain to me how Abbe and Pulfrich refractometers work?
They are such specific types of instrument I haven't been able to find any information detailed enough on the internet or in books.
Does anyone know details on how they work, differences between them or a website or book which explains about them?
Contact details of an organisation/ person who might know would also be greatly appreciated!
Any information would be useful - thank you in advance.

 

[Andy Resnick]

Typically they work by total internal reflection- when the critical angle is measured, the index of refraction can be calculated.