def calc_glass_constants(nd, nF, nC, *partials):
    """Given central, blue and red refractive indices, calculate Vd and PFd.
    
    Args:
        nd, nF, nC: refractive indices at central, short and long wavelengths
        partials (tuple): if present, 2 ref indxs, n4 and n5, wl4 < wl5
        
    Returns:
        V-number and relative partial dispersion from F to d

    If `partials` is present, the return values include the central wavelength
    index and the relative partial dispersion between the 2 refractive indices
    provided from `partials`.
    """
    dFC = nF-nC
    vd = (nd - 1.0)/dFC
    PFd = (nF-nd)/dFC
    if len(partials) == 2:
        n4, n5 = partials
        P45 = (n4-n5)/dFC
        return nd, vd, PFd, P45
    return vd, PFd