Both Optical and Digital refractomers require you to extract liquids, juices and sap from the produce.  

Plastic nosed pliers make it easy to squeeze juices out of fruit or veggies.

Shop here for Plastic-nosed Pliers!  ($15 + Express, Tracked postage).

This is a Japanese refractometer made purely of metal. It is not known whether they used the BRIX scale, but was likely used for a variety of purposes - see below.

For a history of refactory technology, see here. It goes back 4000 years.

For a history of refractometers, see here