It is sight glass. You look in through it. It is dark in there. So, you also need a Light Port.
There are top caps to look down, there are special lights to look past from the outside to see. I have a side glass port to look into.
But, I will use a vertical in-line glass to provide illumination.
For the record. Sight glasses have their uses, but their limitations as well. The Chinese manufacturers say they will withstand 150 psi, but not in any of their official brochures. Accepting that at face value, the ANSI rating would be around a third of that, or 50 psi.
While a system will normally operate under 50 psi, it is possible to hit over 100 psi with heat and the right combined screw ups, and a system should tolerate worst case scenario.
A butane storage tank is limited to an 80% to keep thermal expansion pressures under the 350/450 psi limits on the tanks, so consider if you have your sight glasses on the columns between two closed valves, and the column is full to the brim, that you have hydralic pressures greater than 150 psi. I know of two instances of someone loosing track of what they were doing and exploding their top of column sight glass.
All of WolfWurx, Inc commercial units use high pressure sight glasses in all applications, rated from 720 psi at 250F, and available to 3000 psi.