Assuming that what I am seeing is, in fact, a window (for instance, submarine here) - how strong is the glass used, and is it a weak point compared to the rest of the steel hull, at the pressure of depth? Does a crewman go there to look out the window from time to time, especially if near or on the water’s surface?
The sail is flooded when submerged, so the glass doesn’t have to be very striong.
I don’t understand your explanation because I don’t know what you mean: “The sail is flooded when submerged”. 
Can you please explain?
TIA
The sail fills with water when submerged, so is only accessible when the sub is surfaced.
Sorry, missed the edit window. Since the sail is full of water at the same pressure as the surrounding ocean water, the glass doesn’t need to be very strong.
What I’m getting at is, what is the SAIL? is it the conning tower? or what? I’m not connected with the military so I don’t know these terms.
The sail is the structure sticking up out of the main hull of the submarine. At least the space immediately behind the windowed area is open to the sea (I can’t imagine the whole structure is) so, when the sub submerges, water of equal pressure is on both sides of the glass. No pressure difference, no particular strength is needed.*
Imagine submerging a box made of window glass vs. simply dipping a pane of glass in the water.
*Enough strength to withstand pressure due to the sub’s motion in the water would be needed but that’s much lower than the difference between water at n-meters depth and an air-filled space behind.
ETA: The conning tower is generally contained in the sail but is not necessarily the whole sail.
DesertDog, thank you for your patience with my question and giving me an answer I understood.
What’s the difference?
It’s possible to design a window to withstand the pressure if they really needed to. This submersible has a huge window and can go down to 3900 ft, deeper than most military submarines.
Per Wiki:
As improvements in technology allowed the periscopes to be made longer it became unnecessary to raise the conning station above the main pressure hull. USS Triton (laid down 1956) was the last American submarine to have a conning tower. The additional conning tower pressure hull was eliminated and its functions were added to the command and control center. Thus it is incorrect to refer to the sail of a modern submarine as a conning tower.
[ninja’d]
Why does a submarine need a sail, anyway?
Beats rowing.
This is the right answer.
Yes but that may not fair well with underwater explosives.
Seriously, though. Why does a submarine even need a superstructure? Why can’t it just be a smooth, symmetrical cylinder? Seems to be that taking the lump away would make it more hydrodynamic and stealthy. So what’s it there for?
It could be as you say.
The reason is to have something that sticks well out of the water when surfaced.
If it was a bullet shaped tube no one would be able to exit the sub except in the calmest of water.
Okay, let’s go one step further. Why doesn’t the lens in the periscope implode? Is the periscope pressurized with the rest of the sub, or is the lens just that tough? How about the beacon, etc.?
Periscope lenses are pretty thick and tough and aren’t likely to implode. Besides, nowadays periscopes are replaced with electronic masts that basically have “cameras on a stick”
The periscope was an incredible piece of engineering – it wasn’t just a lens at the top and a lens at the bottom with a couple of mirrors to redirect the light. That whole tube was crammed with relay lenses and the like. Each lens had to be anti-reflection coated, or else Fresnel losses at that many surfaces would cause all the light to be reflected away.
Add to that the fact that if you use a periscope while the sub is underway it will be pushed backwards by the drag of the water. This can cause the whole thing to go out of alignment. And that at all times you want to avoid humid air from inside the sub getting into the periscope tube as well as water from the outside – because if those intermediate lenses fog up you won’t be able to see anything. So they pack dessicants in there, too.
A lot went into classic periscope design. Even though the periscope was invented before submarines became practical (they were used to see above the tops of military trenches without getting shot), it was a long time before all the problems were addressed. No wonder Jules Verne didn’t put a periscope on his [Nautilus.