sight hole

基本解释

The fracture phenomenon and spalling time of the test piece can be observed continuously through a sight hole.
通过窥视孔可连续观察试件表面的脱皮现象和得出激热开裂时间。
Glowing dots naturally gathers light to surround peep sight hole allowing huner to visually locate and center peep in dim light.
发光点利用自然光包围窥孔的方式使窥孔产生高亮，达到射手在昏暗的光线中超清晰的使用。
bow123.com
It features that the sight hole of sighting tube made of graphite is fixed to inner hole of shell to form an optical measuring cavity which only has an aerating hole for filling pure gas in it.
石墨窥视管与壳体固连后由石墨窥视管的窥视孔和壳体内孔对接形成了与外界隔离的、结构上呈封闭状的光测腔；
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But even as we dig our way out of this deep hole, it's important that we not lose sight of what led us into this mess in the first place.
WHITEHOUSE: The “Volcker Rule” for Financial Institutions
Thus, the outline of the black hole would be detectable as they move across our line of sight.
FORBES: Free-Floating, Nearby Black Holes Detectable By Decade's End, New Study Reports