A choice of two from

How it works: Grab two books from a trilogy of your choice in each hand.

怎么做：从三本书里选两本书抓在手上，一手一本。

The choice of proper initial Angle of incidence makes a lateral shearing interference easy to realize between the two beams reflected from the front and back surfaces of a plane-parallel plate.

通过选择合适的初始入射角，使从平板前后表面反射的两光束实现剪切干涉。

Units can be constructed from a single alarm channel to a maximum of 128 channels, in any format, with ultra-bright LED illumination as standard and with a choice of two window sizes and six colours.

模块可以组装成任意结构，从有单一的警报主频到最多的128警报主频，并且有超亮的LED灯彩标准以及两种窗口型号和六种颜色的选择。