@atomicker

So I did a bit of digging and found that the mentioned island (材木島/Zaimokujima or Timber Island) is at coordinates 38.33004, 141.06947. Now you can find it on the map.

However, the arc has collapsed since then and no longer exists.

Here's an old picture where it can still be seen, dated by the Tohoku University Archives to be from the 1910s or 1920s.