ophthalmoscopy affords the only opportunity for direct visualization of blood vessels . 检眼镜检查法提供直接观察血管的唯一机会。
such products may represent physical quantities, but at present our thinking is merely formal and not attached to any physical visualization . 这样两个积可以是物理量,但目前我们的想法只是形式的,并未赋予任何物理直观性。