Porthole is commonly used in the realm of boats and ships. It's nautical vocabulary.

Maritime Terms

Unlike typical windows, portholes are generally round.

Circular Shape

Portholes can be opened for ventilation but are designed to be watertight.

Safety and Function

Porthole can refer to similar openings in vehicles other than ships, like spacecraft.

Not Always Maritime

Porthole is often used in technical and professional settings, particularly in transportation.

Technical Sphere

Porthole suggests a smaller size, hinting at limited viewing space.

Size Implication

 They’re the portholes through which light enters the eye, where it then strikes the retina and activates light-sensitive cells, setting the process of vision in motion. Pupils are black because most of the light that enters them is absorbed.

Why do cats have...

 that time to take pictures there so what Beebe did is he called on a telephone line up to the surface to artist bosserman on the surface and as he peered out of the portholes in the deep she was up there drawing and painting these

The New Era of Discovery...

 is that it's preposterously wasteful. It postulates a vast and rapidly growing number of universes existing in parallel, mutually undetectable, except through the narrow porthole of quantum mechanical experiments.