原话是 : Form a cross-over spot on the screen by the brightness key.Depress the dark brightness key and present a beam spot of about 100 mm dia. on the screen and call it an under-spot.Get back to the cross-over spot once again.Dpress the white brightness key and form a beam spot of about 100 mm dia. on the screen and call it an over-spot. 接下来的步骤是,调节以上两个点,使它们重合,形成同心圆。我没能理解它的意思,更本没有发现以上两个点,其他得更谈不上了。