The "Gauge glass" is the device that displays the water level in the boiler. It can be thought of as "in parallel" with the try cocks (three or four) arranged in the backhead above and below the proper water level line above the crownsheet of the firebox. The gauge glasses have valves in the backhead at both ends which are positioned so that one is above the correct water level and the other is below it. There is a third valve below the lower backhead valve which can be opened to allow steam to "blow" the water out of the glass and then reclosed so the level reading remains accurate in the glass. These are covered by a secondary housing with glass windows it it to view the gauge glass proper through in case it breaks, which sometimes happens.
"Sight" glasses are the little round glass windows located in the side of a hydrostatic lubricator where you can observe the flow of valve oil
through the unit. There are tube-type "sight" glasses in the side of the oil reservoirs of some mechanical lubricators, and also they are often found in the sides of diesel locomotive fuel tanks to show the fuel oil level in the tanks.