def H_osc_str(n,delta_n):
    """
    Oscillator strengths for hydrogen for n=50-150,
    delta_n=1-5. Taken from Menzel, ApJSuppl 161, 221
    (1970)
    """
    # The intercept for the formulas below are fit quite well with
    # 10*n**-2.85
    if delta_n == 1:
        result = 9.8 + 9.6*(n-50)/50.
    elif delta_n == 2:
        result = 1.4 + 1.31*(n-50)/50.
    elif delta_n == 3:
        result = 0.44 + 0.41*(n-50)/50.
    elif delta_n == 4:
        result = 0.195 + 0.175*(n-50)/50.
    elif delta_n == 5:
        result = 0.105 + 0.090*(n-50)/50.
    else:
        result = -1
    return result