[填空题]
Two (and only two) full specusfpi:y i4n m:))*kd*p(tl)lo k cu4itral orders can be seen on either sak8l p+biun1)-mvqw3ide of the central maximum3bnq1k uil wp)vam+ -8 when white light is sent through a diffraction grating. What is the maximum number of lines per cm for the grating? $ \times10^{ 3}\;lines/cm$
