What is a “tube lens” in a mass-spectrometer ion source and what does it do?

I came across the Russian phrase "Transporting capillary voltage: 85 V" in a text I was translating. The paragraph was describing the operating parameters of an electrospray ionization-based ion source in an MS system.

The author told me that the proper English translation is "Tube lens voltage: 85 V". I googled for electrospray "tube lens" and found this picture:

I haven't found a down-to-earth explanation of what it is yet. Maybe someone here knows and can explain this in simple words.

Why is it a "lens"? It looks like a simple channel throught which the liquid flows to the needle.

• By the time it hits the lens, the liquid had better be a stream of ionized molecules and atoms. Then it looks just like the first lens in almost any ion source including the skimmer as the ground of the electrostatic lens. If a molecule is ionized, consider the field lines between the tube at potential and the skimmer at ground. – Jon Custer Oct 10 '16 at 15:18