sub Axis {

#2 sets of cartesian XYZ co-ordinates on the axis
($Xl1, $Yl1, $Zl1, $Xl2, $Yl2, $Zl2) = @_;

$Axx = $Xl1 - $Xl2; 
$Axy = $Yl1 - $Yl2;
$Axz = $Zl1 - $Zl2;

$Vform = V($Axx,$Axy,$Axz);

$eq = sqrt($Axx**2 + $Axy**2 + $Axz**2);

$Unitvector = 1/$eq * $Vform;

return($Vform, $Unitvector);

}